The key thing is that summoning a demon instead of a loa shouldn't be stupid. You shouldn't have to be compelled into it.
You summon different entities for different things. Most likely, the player says, "I want to summon something for this purpose," and rolls Lore to figure out what it is. Maybe a high Lore roll gives him something he can bargain with and thus get his results at a lower complexity, while a low lore roll means he has to go for something that's going to be a pain in the ass to deal with.
And if you let people just say "I'm summoning something friendly" and make things easier for themselves, then summoning a demon instead of a loa is stupid.
If the player is naming specifically what he wants to summon, the GM still has the final say, and could easily say, "Okay, knowing the name of a Loa is an 8-shift Lore difficulty, but you have this book of demons over there and it's a Lore difficulty of 4 to summon something from there." Or maybe the player spends a fate point to declare it knows of something friendly that can help.
It's not like the Player has the final say in all this, remember.
I dunno about more powerful...but most friendly things aren't combat oriented. 'Friendly' and 'combative' not going together as often as some other personality traits...
I meant friendly to the summoner, not friendly in general.
At the same time, though, friendly can sure as hell be combat oriented. Michael Carpenter, Sanya and Shiro are plenty friendly.