Focus is great and works less buggy than non-focus methods I have found (at least in my limited experience). However there are times that you want to use focus for something other than dpsing the current main target. Since I roll with 2 mages and I want to be able to sheep independantly of the dps target, I reserve focus for CC targets. I feel more comfortable with focus in this roll with this team. If I was to try a different 5 team I might go back to focus. I think it really comes down to which you feel more comfortable with. Also if you are rocking the 5 shammy team where CC is not really a requirement, there is no reason to not use focus for the dps target.

Hope this helps a little.