That thus depends fully on the DM/group style if that works or not, and imho that's not balanced! In the social setting the the combat person is useless, and in the combat the social person is useless. You're essentially playing different games, with different people, and the rest of the people wait for you to finish the others game before you continue their game. This tends to be not fun for all involved. And the more sub games you have the longer the wait becomes. This is the same issue as splitting the party with a single DM, running different games while others wait around.
There should be player/character involvement in all aspects of the game, everyone should be able to participate in combat, in social, and exploration. And this is what I mean with building characters as a group, not to have an expert in each field, but to make sure there are no extremes and no 'holes' in both the group AND the characters. It's also to get everyone in the same mindset for the adventure/campaign.
I don't really agree with this, mostly because people are not useless typically. While one character might be better at combat and another better at a social encounter, both can usually contribute in either.
When it comes to designing characters as a group, that works sometimes and does not work others and is largely player and group dependent. I will say that personally as a player, my character is usually built for me, not for the party. If others want to build to make sure there are no holes in the party, that is fine but it is on them and their character to do that because I am going to play what I want to play and I usually have that figured out before session 0.


