Seth Godin started a thread (see: Thanks) and Rob Safuto of Awakened Voice picked up on it (see: Losing Your Users). In summary, Seth points out that blog [and podcast] followers give their creators the gift of attention, and Rob follows that up with a thought that ad services are a specific and directed invitation for the audience to go away.

Basically, if you want to attract a following and you respect the people that give you their time, why would you push them out the door?