This is funny in a funny-sad way. You can search WordPress.com for tags that are single words, but not multiple words separated by spaces:
- http://wordpress.com/tag/spire returns results.
- http://wordpress.com/tag/viro returns results.
- http://wordpress.com/tag/spire+viro doesn’t return any results.
- http://wordpress.com/tag/spire%20viro doesn’t return any results.
I’m trying to think of a valid reason why. Identifiers can’t have spaces in the WordPress.com backend system? Hello, DOS!
The Pursuit of a Life 
Hello there,
We haven’t met previously. I have been a WordPress blogger for going on 6 years now and I answer questions on the support forum. Tag search is not “broken”. You simply aren’t aware of the search format instructions provided in the support documentation so you know the format to use that will produce results. See here please > global tags http://en.support.wordpress.com/global-tags/ I hope this helps clear up any confusion you have and wish you all the best with your blogging.