I have also DMed many sessions that were nothing but roleplaying scenarios. The thing is, it was moving the story along. Talking about turnips is not moving the story along unless it is actually part of the plot. If you still don't have a problem with the group spending an entire session talking about turnips, which has nothing to do with any plotline, and you guys enjoy it, that's great.
I just don't think it makes much sense to claim that turnip-talk is an acceptable way to spend a gaming session, but adventuring in the Underdark is a waste of a groups time.