I should note that the math shows that Edge vs Oversummoning is the only real factor balancing it out. As for the rule not being there, it wasn't there in SR4a either, it was a rule (not optional, simply a rule) from a supplement. Using the trick in advance to help balance things is a perfectly-legal GM tool. The no-Edge for Spirits is merely to avoid players arguing that the Spirit should use Edge to execute the task the player gave it.
They have edge while being summoned, that much is uncontested, but why would a spirit use it? I mean, spirits like being in the world and they have no clue who is summoning them or to what purpose. Resisting binding would make sense in the game-world, but summoning? Unless a spirit is summoned by his true name, which spells slavery in capital letters, I don't see a real reason why a spirit would resist.
Plus, isn't being lopsided in favor of the summoner kind of the point of summoning? The magician is kinda supposed to get hits to get services. The more powerful the spirit, the less likely he is to get more than one service. I mean, the force alone would cancel his magic attribute and eat into his summoning skill.
I'm all for preventing imbalances, but basically telling the player "I'm going to alter your assumptions about the game by introducing another element that gives your opponent an edge over you" is kind of a heavy-handed way to do it. At that point, you might as well fudge the dice.
In that sense, I'd like to point out a GM's tool that has been neglected here so far: (Only applies to bound spirits)
If the gamemaster chooses, a spirit that has been set on a particularly long and/or undignified task may struggle against their binding, which imposes a –1 penalty to all tests as the magician works to compel the spirit to do his bidding, like a dog on a leash (an analogy, by the way, that would not make many spirits happy). At any point, the magician can take a Complex Action to try to bring the spirit to heel through an Opposed Summoning + Magic vs. spirit’s Force + Willpower Test.If the magician ties the spirit or get more hits, the spirit is calmed down and performs their service without further penalties for the summoner.
From a lore perspective, I can easily see a powerful spirit (aka one that exceeds the magic rating of the summoner) resenting being ordered around. A weak summoner is likely to fail that test, given that he's basically facing double the spirits force. It's not a large penalty, but it sticks around unless he expends a complex action.
As far as unbound spirits go, spirits are compelled to perform their services but how they go about this is entirely up to them. There is no rule compelling an oversummoned spirit to fight reckless or inefficient. If a spirit is so powerful that he cannot be harmed by the enemy, its entirely reasonable to assume that he feel that he can also take his time and enjoy it. What I'm saying is basically, spirits think different from people and they have their own motivations and understanding of things. A powerful spirit with mental stats way beyond its summoner can easily force his master to think their orders through *very* carefully if he wants to. He can also quite easily play dumb and force his master to expend more services than he intended to by giving new orders.