Why? Bcs your theory is based on wrong presumption ...
Your theory is only working with the idea of this game having 6 characters, bcs right now there is only 5 Origin characters + Tav ... once Origin characters will become playable base of yout theory that "its's already build for a party of 5 or 6" starts to fall ... but not even close as much as how it starts to crumble once they start to implement other Origin characters ... I mean we kinda "know" there should be at least 3 more (Karlach, Minsc and Helia) ... that would mean that the same XP that by yout theory "was ment to 6" is sudently "ment for 9"? Just bcs you still cant accept the fact that we get theese Origins just bcs they were first done, not bcs they are only one, or somehow most important. -_-
Also how would you like to level your inactive party members once you would drag them for their quest? I mean you will have Tav + Gale + Shadow + Lae'zel + Astarion + Minsc ... all of them being level 5/6 ... and then Wyll (who you never used before) wants to join you, not for some Goblin party ... How is that enjoyable model to drag useless lvl 1 with you just bcs you wish to do his own quest? Or would you need to switch them regulary so they all have aproximately simmilar power?
And finaly and most importantly ... What benefit exactly would this system bring? I cant find any ... yes, your "main party" would be exping a little faster (presuming you will still split the same amount of XP, wich i also doubt) ... but your "off party members" would be next to useless. :-/