The main issue with auto insertion IMHO is that you end up putting it in the very beginning of a show, and destroy branding.
I don't want the 1st voice my subscriber hears to be someone else. Putting them in the end is problematic too as people tend to skip by them or never hear them if a shows final topic (ie the feedback/email part doesn't interest them), and in my show I have exit music that trails off for 15 seconds, so it would sound a little out of place.
I start out all my shows with me saying that my show is sponsored in part by <insert advertiser>, its 10 seconds, its my voice, and the 1st thing you hear is 'The Mike Thinks Podcast is brought to you by'....so you hear my brand right up front, in my voice.