You need to stop thinking of Regen as a healing spell, and start thinking of it as a "buffer" for the tank's health.

Sure, you can use it on other members of your party/raid, but in general, it's meant to be sitting on the tank, making his/her health spike less.

Personally, when I'm healing as a White Mage, I only use Regen on packs of at least 2 mobs, and I stop putting it out once the second-to-last mob is nearly dead. That way, it won't be of use to the tank during the highest point of damage (when there's multiple mobs on him) and won't be on the tank when the last mob dies (thus enabling us to move to the next pack of mobs more quickly).