It's ultimately personal preference, but there are definite advantages to having the current kill target as an actual target.

The most compelling for me is the ability to deliberately attack a target other than the main's target.

I often run around looting with the main while the drones keep killing, or swap targets to generate threat and I want the DPS to continue on the kill target.

Then there's the ability to use the reset=target macro construct and the shiny new interact with target.

Oh, and the fact that I don't need to make a macro for every single damn spell I ever use!

I've never used focus for anything other than secondary abilities like pet tanking (where I can assign a different target to each hunter / warlock) and sheeping.

So, my system is - and always has been - based entirely around follow and assist. I wouldn't change it for the world - it's really not hard just to do a quick visual on the party targets to make sure they all have the right raid icon when you really do want focussed fire