Nice application,it looks interesting !
Coming to your question, I would say the sidebar is highly appropriate in the following cases :
1) When you have content which you want to be visible at all times or have links which serve as redirects to specific sections within the site
2) When you want to provide localized content to the user about stuff he is interested in
3) Most of the languages in the world read from left to right,so if you are providing user specific information which should be always available to the user at first,having the user profile at left allows the user to quickly glance at relevant information which is applicable in his case (as per Jakob Neilson users generally read in a F shaped pattern).For example, taking your own mockup it would be the picture,the relevant tags,recent posts and in case since the person subscribes on basis of tags,a subscribe box for the tags