Not entirely wrong Mirikon, there is actually quite good basis for the ruling that your tradition dictates what you can have a spirit do in addition to what sort of spells they can affect.
Street Grimoire has a rule in it that actually says that. I forget where exactly, but I know it has been discussed quite a bit in the past.
The rules are quite clear on how the spirit category dictates what sort of spells the spirits can aid and so forth, so we don't need to debate that aspect. However I will point out that for Dedicated Conjurer, this means the new spirits don't have a category, and therefore can't aid sorcery.
As far as the other part goes, personally, I go for a bit of a compromise, and the way I handle it actually helps bring the roleplaying in line with the setting and works to prevent simple power-gaming (the "I summon this spirit because it is better than the others" mentality). Simply put, the category of spells that a spirit governs also shapes your view of what that particular spirit is meant to do.
So, for example, a Shaman plans to go into combat, so they prepare by summoning a Beast spirit. That's their Combat spirit, so it just makes sense. You want to use a spirit to help you find someone, summon your Detection spirit, you want to hide? Illusion spirit. You don't necessarily summon those spirits because they can't do other things, but because that's their purpose from your magical stand-point. Now, there are always ways to twist that concept to move things along, such as using your Detection spirit to Conceal the team because you are "evading detection" so that still falls in that spirit's sphere of influence. Or my personal favorite, asking a Health spirit to "Protect these people" which means that it will engage in combat to defend the lives of those individuals.