The trouble with not sharing social skills is that one character will end up doing all the talking (and buying/selling) while the other sits and watches. This isn't much of an issue in single player, where the logistics of it don't really matter, but it's not particularly multiplayer-friendly if one player gets stuck twiddling their thumbs all the time because the other one has the social-skilled character.
Nor is it a good idea to expect both characters to crank their social skills because - apart from putting a serious damper on character concepts for your two-person party - it's also largely redundant in single player, which means that it ends up as a 'multiplayer tax'.
Either one of these is particularly bad given the design intent of drop-in multiplayer.
When you say, leveling up social skills, you're talking about only the barter skill correct? Because sharing the entire social tree would completely ruin the in-game party discussion role play element.
Besides, your argument for the lack of interaction during trade falls flat. Some monsters you can't dispatch with melee attacks, sometimes you need to pick locks or sneak. Should the player who didn't level up their sneak/lockpick/dexterity be subjected to the mind dulling boredom of passing turns and not interacting with locks? If you want your character to excel in something you need to level it up.
All in all though, if I'm chillaxing with Vince Shlomi, the shamwow guy, it doesn't make me a good people person. The same way as if Legolas is chillaxing with Aragorn, Aragorn doesn't become a master archer.
