Why the "ten party member" goal? If you're going to do it, support arbitrary size!

[The Fred]

This is an engine. The fewer the hardcoded limitations, the better the engine

This.

 

It just opens up the possibilities. I also see the potential for, say, a mini-quest where you team up with someone, without having to ditch another party member and start reshuffling inventories and things. They wouldn't necessarily be permanent but you'd be able to have them when you wanted/needed to.