I am usually placed in group with the tanks. Any other spots in the group are usually given to warlocks to allow them to tap with slightly less burden on the healers.
That little trickle of heals isn't much to the tanks and is, in most cases, bordering on useless. But it may just buy your healers an extra split second of react time to keep someone up incase the tank took a couple big hits or his healer lagged, got stunned, whatever. Use it for the same reason you use food buffs, a little extra insurance.
The game is about min/max'ing on cutting edge content for your gear level, VE is one way of doing it. When your raid is geared to the teeth, you probably don't need it. But when you are starting out and undergeared for content, it can help.