I got drawn in by this question, and haven't read anyone's responses yet. Here goes a bit of a ramble...
I think it depends on Adam's (and/or Let's Talk Bitcoin's) cash flow and the long-term plans. If LTB is a start-up, and @Adam / LTB can afford this to wait awhile to worry about profit, maybe it makes sense to keep it free and expand the listenership as much as possible. Try to win on both content quality and size of listenership, and just try to own this segment. However, if there's a need for cash to pay the bills (personal and/or business), then I could see changing the model out of necessity.
The clouding component to this, in my mind, is the LTBcoin experiment that's integrated into the podcast network. There's the experiment of paying content creators, contributors, and the like in LTBcoin. There's the experiment of rewarding content consumers with LTBcoin. Then there are different types of coins that have been created and pegged to advertising credits and Skype time with Adam. These tokens are auctioned off, and may be being purchased speculatively, as well as for use. It's one hell of an experiment, and one hell of a complex system. Changing to a walled content garden seems like a big change to the direction this has been going. I think a lot of people prefer to try a free model, and have a wider listenership.
The anchoring of LTBcoin to advertising value seemed important to me. But that's been decoupled a bit, with the advertising tokens, which I think are floating around, and not being used at the rate they are being sold (for LTBcoin)… I also think that advertisers encounter a lot of friction, in that they can't just buy an ad from Adam or Let's Talk Bitcoin. There's a confusing layer of having to go through an intermediary, who bought ad tokens in bulk by playing the winning bid on the LTBAuctioneer page. The longterm goal, I believe, is for small agencies to form, buy in bulk, and be an outside ad sales department (I know that's happening, but not sure to what degree). And for LTBcoin to have a steady or increasing value. Maybe the agency formation will be successful in the long term. But for now, I think advertisers may just not have a clear idea who to go to and/or how to buy an ad, and that may be revenue lost. Is there any site/podcast that's harder for an advertiser to buy an ad on? Would be great to reduce friction on this, have links that go directly to any agencies, or people who have ad tokens to sell.