Note I'm trying something new here. Rules citations are blue, poster's comments are normal quotes.
No, you are reading that wrong. Plus - what the point of the summoner being able to mentally communicate with the spirit during combat if he can't direct him to do something.
To be technical, the summoner does not communicate mentally with the spirit. Its the other way around. While most GMs handwave this to mean a two way link, the rules are quite clear that it is one way. The summoner only knows when a spirit is disrupted because of the loss of the link. The spirit can keep this link open to receive orders, but it is born from the spirits powers, not the magicians. So technically, a spirit can chose not to listen by closing the link, forcing the magician to talk.
A spirit doesn’t have to speak to his summoner out loud. It can communicate telepathically with the summoner, even from astral space, so it doesn’t even have to manifest to receive orders or make reports. This link allows for communication over a distance but does not extend to the metaplanes, nor does it allow any other visual or audio connection. With this link, a summoner knows when a spirit he has summoned has been disrupted, as he will feel the loss of the link.
(P. 302 Core)
Power Use is a completely separate service from Combat - which is why the clarification is there. Why give a Spirit of Man a fireball spell if you can't tell it to cast the spell? Or Heal? Or whatever.
A few things here. First RAW:
Services are how we measure how helpful a spirit is willing (or required) to be to you. A service is a single task that you request (or demand) from a spirit.
(P. 302 Core)
This passage is followed by the list of unbound and bound services. These lists are exhaustive, meaning there are no other services than these. Among these is the "Power Use" service, which reads as follows:
Power use: You can have a spirit use one of its powers on a target or targets of your choosing. If the power is sustained, it counts as one service no matter how long it’s sustained. If the spirit uses a power as part of another task (often in combat), then the power use doesn’t count as a separate service.
(P. 302 Core)
(Emphasis mine) The service says very clearly that it counts as a service, by virtue of being part of the list. It goes on to state that if the spirit, not the summoner, decides to use a power, it does not count as a service.
Now lets extend out scope a bit:
Conjuring is the art of calling, dismissing, and controlling independent astral beings called spirits. Conjuring can compel a spirit to come to the magician and provide services or favors with Summoning, press a spirit into lasting service with Binding, or dismiss or destroy a spirit with Banishing.
(P. 299 Core)
Spirits are independent beings with the sapience power.
Sapient critters are self-aware, capable of making their own choices, and are generally at or above the level of Homo sapiens.
(P. 400 Core)
Sapience is defined as beings with the ability to make their own choices. Spirits, like any other NPC, are under the control of the Gamemaster. Knowing this, we can answer your question:
Power Use is a completely separate service from Combat - which is why the clarification is there. Why give a Spirit of Man a fireball spell if you can't tell it to cast the spell? Or Heal? Or whatever
You absolutely can tell it to use its powers, but if you do so, it costs a service. The key to your supposition that combat overrides the normal spirit rules lies in a (I think) misunderstanding of how spirits work. Spirits are sapient beings, as established above, they make their own choices. When a magician summons a spirit, he basically enslaves it to his will to perform a number of services. Note the terms "compel" and "press into service". A spirit can fight this, but the magician can bring him to heel (rules P. 301 Core, "Bad Feelings With Bound Spirits").
The GM makes decisions for the spirit as he would for any other NPC. Within the world, the spirit makes his own decisions. When the summoner overrides the spirits will to compel a service, he pays for this with a service owed. At the game table this means that despite the fact that the GM has overall control, the player can override the GM and demand the spirit do something else. If the service asked is to do battle on behalf of the summoner, that costs one service and the GM at the table and the spirit in the gameworld decide how best to go about it. The summoner however still maintains his control and if he does not like the spirits actions, he can order it to do something else. However, doing so overrides the will of the spirit/ GM once more and therefore, costs another service.
There is another important implication of that rule, and that is that a spirit cannot free itself by using its powers during combat. If that formulation wasn't there, a spirit could simply cast a few spells, each counting as a Power Use service and then leave once its services ran out. With the rule a spirit can still "play dumb" to coax the summoner into giving it direct orders, but its the magicians choice whether or not to do that.
In short: a magician can order it to do any service he wishes and if the spirit decides that end is best met through the means of a power, it does so of its own free will. But of the magician orders it to use a power during the process of another service, it overrides the will of the spirit and therefore costs a service.
The way you interpret it, is basically the old wishing for more wishes scenario. You want the spirit to pretend that your first override of his will, to go into combat for you, gives you licence to continually override it's will until "combat" ends. What's to stop a magician from defining combat as his struggle for something? Nothing, but it doesn't matter because it's the spirit that decides what constitutes a service and what doesn't. From a game-world perspective, the rules are little more than "tried and true guidelines".
In order to lighten the mood of this topic a bit, here's a fun story about how spirit services can work in the game-world: (2nd edition)
A long time ago, we were facing off against a recuring villain magician in a climactic battle. This magician, apart from being a first class bad guy, had a habit of abusing spirits. On more than one occasion, he had ordered low force fire spirits to take a swim, water spirits to mud wrestle and the like, but he also had a habit of using low force spirits for purposes of grounding. During our final encounter, he was accompanied by a force 13 fire elemental. Our magician burned and used almost his entire karma pool to take control of the elemental and barely managed to do so, gaining control and one service in the process and was almost incapacitated by the drain. Right after taking control, he ordered the elemental to "burn the fucker". The elemental promptly proceeded to engulf the enemy mage. Shortly after, the mage turned to ash and his support was mopped up. Then the elemental bowed down to our mage and asked for his orders. Our mage was somewhat confused and told him he had only had one service, which he had fulfilled. To this, the elemental smiled and said: "Yeah, I know. But that bastard was for free."
Moral is, the rules may say how spirits work mechanically, but they also warn you to treat them nicely. Word gets around and there are spirits out there more powerful than you. For all we know, their home-plane is perfect society where everyone loves everyone and the spirit you just tried to con into giving you more services than it owes is little brother to a legion of high force spirits, some of which are bound to your enemies. Spirits make their own decisions and when ordered into combat, they may just go straight for you next time.
As the saying goes: "Don't shit where you eat." Spirits can be your most powerful ally or worst enemy. Abuse them at your own risk.
