Taking the lane is not always possible or desirable. The situation I was in yesterday was such an example. It was a very congested street, with no room for a bike to take the lane. Even some vehicles were going between lanes. A very hectic scene that I'll try to avoid in future.
I don't see it as being 'desirable', or 'undesirable'. I see it as a necessity. I have ridden in PM-Rush traffic numerous times, 'taking the lane' each ride. Because, It keeps me safe. I don't ride in a BL or DZBL because, I don't trust motorists' to, not to be stupid and stray into the bike lane. I know some are going to think I am nuts. But in Maryland, signs were finally posted by the state DOT, telling motorists' that a cyclist may use the full lane.