You're thinking like a developer. 99% of users would never use something like this. No way in hell are they going to learn SQL to use the search system. Something simple like how Google does it would be much better
Something like: keyword member:Eriksrocks or foo bar member:Eriksrocks tags:Google,XenForo,Facebook
Sorry, but the idea of SQL in search queries just makes me want to cringe. The only place this would be useful would be for modifications, in which case you can just use standard SQL.
Well, I can see how this could be useful if you wanted to provide SQL-like functionality while restricting what can be queried, retrieved, etc. Essentially if you wanted to limit the SQL functionality. Though, for the reasons I mentioned above, it's not that useful for search. And yes, it would add extra overhead.