As an end user, I find blocks cumbersome. I really don't see the point in complicating blog posts - which is really the only aspect of my site that I'm adding to regularly.

For other pages (home, contact, etc) I see nothing wrong with theme specific blocks or templates. Yes if I change themes I might have to fix a few pages, but if I'm changing themes I'm probably going to restructure my site anyway (otherwise why bother with a redesign).