Bitcoin already has a large population of users who have little or no technical skill, it wouldn't surprise me at all if it was found to be the clear majority by now. Assuming success and growth in future, very few users will make any decisions at all about their privacy, they will just accept the defaults. In such a world no consumer wallet is going to directly expose Tor to end users - if used at all it'll just be used behind the scenes. So automated fallback or control over exits would be a concern for such wallets.

My gut feeling about this stuff has changed over time. I don't think it'd be a great idea to tie Bitcoin to Tor too deeply, convenient though its infrastructure is. Most apps don't need a whole lot of onion routing - a small amount built in to the p2p layer would be sufficient. Tor is huge, complicated and could be a liability in future.