You should NOT tunnel vision like that in PvP.
Just one simple example for why hiding all other nameplates is stupid: You tunnel vision your selected target, while there's at least one other LOW health target nearby that would have been dead if you switched.
Seriously, if you only want a "nameplate" for your selected target you could aswell put your target frame in the middle of your screen and don't use nameplates at all.
Besides, having to use un-throttled OnUpdates for such a stupid thing is a waste of resources.