I'm not really arguing that forcing tags into community tags was a bad idea, I had a very similar idea a week or two ago and was arguing that community tags were useless without forcing it in there in some way.
You can see that elsewhere in this post, one of the biggest complaints is the feeling of clutter. Having two primary and two corresponding personal tags was a mess.
That's primarily a display issue, though, that is easily solved.
Right now we see this on this post:
"text · #blog · #blog.hubski · #hubski"
All you have to do is detect that #blog and #blog.hubski are in a row and simplify the display to:
"text · #blog.hubski · #hubski"
Or something. You could even have #blog.hubski be two links, one for #blog and the ".hubski" part link to #blog.hubski". That, I believe, is most people's concern with visual clutter. It's just redundant display. Once we get to #blog.hubski when reading things, we already know it's in #blog. We also know it's already posted by hubski, so maybe something like:
Where the "@" is what gets a second link.
I dunno I'm just throwing stuff out there right now.
One thing I have also considered is how this plays into scaling. The conventional wisdom says that as tags grow more popular, the ratio of quality content will fall. However, what if I really want to get great #politics posts even after hundreds of users are submitting to it? Previously, I would have had to follow #politics, and filter an ever-increasingly large swath of users (in entirety, not just their politics posts) to cull out the chaff. It is a race, that eventually I would lose. Now, I have the option to browse #politics, find the best users submitting to it, and follow their personal politics tags. With this approach, my politics feed doesn't degrade over time. Of course, I need to work if I want new voices, but I won't have to be constantly filtering users to have a quality politics feed.
I guess my point with keeping a second tag that is forced by the poster actually helps not in user discovery, but tag discovery. For instance, I would have no idea #privacy or #surveillance existed without double tagged posts.
Then, it does help with user discovery since now that I have #privacy and #surveillance followed, more posts are going to show up. Someone might tag #uspolitics and #surveillance, but say I'm not fond of #uspolitics. Mr. US Politics subscriber feels that as a community tag, #nsa is a better tag for a post, and I never even see the post or know that user exists.