I've always been an advocate of a single feed and site for each podcast. A lot of podcasters out there use an existing blog as the platform for their show and there are some problems with doing that.
Readers and listeners aren't necessarily the same people
A lot of podcaster/blogger types figure that their listeners and readers are the same people. That may be true if you're definitely only ever talking and writing about a single topic, but most of us aren't.
My personal blog and my podcast each have their own site. There's certainly a percentage of people that read my blog and listen to my podcast, but it's a pretty small percentage. Both do all right in their own right, but they hit two different audiences.
Another reason is stats. Donna Papacosta was pondering a similar quandary about stats recently. While having a separate personal blog and podcast wouldn't necessarily solve her specific problem, it might help separate her two different types of audience.