Really? My god if a tank can't manage more than one mob they must be pretty lame.
Sorry but it's true. Even if you can't CC there is more than enough abilities across all of the classes in this game to ensure the tank can maintain aggro.
Guard on DPS, ALWAYS
As for damage, someone has to take it. If you guard the healer the DPS you just took it off will now get that extra 5% (woooooh big damage!)
I play Black Hole geared marauder as my main and do FPs on a regular basis with group finder which usually pops me into a group with a melee tank around rakata gear and a another ranged dps. I still steal a lot of aggro from the tanks because of my rather high end gear and big dps. I don't say it's completely wrong to guard the healer but if the healer is already decently geared(columi level or around that) then I would like to get the guard because that makes the run so much more easy as I don't have to use my defensive CDs so much and tune down my DPS but I can only concentrate on the thing I do best, the awesome dpsing. It's so much more easy for me and the healer in the bosses that have multiple enemies(Boarding Party, Kaon...) when I have the guard so the healer does not need to heal me so much even though I tend to steal the aggro every now and then even with the guard, just because my dps is so high.