Honestly it dont bother me at all, since im kinda used to it ... In most RPG i know you have limited party with no story explanation, since its pure mechanic thing ... the question that is more pressing for me would be what will Larian do with unused companions?
I must say i certainly like how they use them in some games ... Either that in some point your character is indisposed, and rest of the group need to help him/her ... or that you split to two groups where each have its own battle to fight.
Much better than let "unused" companions just stick in the camp the whole time. Its like "Oh did you know Hero of *XY*?" > "Nah, i was just maintining his fire in camp, while he saved the f*cking world."
Examples: Dragon Age: Origins, or end of Mass Effect 2 comes to my mind ...