So if you are fairly sure what party you will use 90% of the time, is there really any reason to not have the option that only the active party members gets xp?

well one reason would be if you want to juggle companions for variety. also you would shoot far above what you should've had in terms of levels - that might make the game easier than you'd like.

in terms of raw power - yes. it is by far a lot better to turn off shared exp.