Disclaimer: I'm in a mostly casual raiding guild -- we expect progression but don't have the attitudes of most of the serious raiding guilds. This is how I like it, so most of what I'm saying is from that perspective.

As a former holy paladin that rerolled to a resto shaman, I understand your pain. However, not all other healers are healing the tank out from under you on purpose. As an example, our guild is pretty heavy on melee right now. This leaves me throwing a lot of chain heals near the tank and chain heal automatically targets the nearest people that need the most healing. It's going to hit the tank and there's nothing I can do about it. I can't stop healing the melee when I'm assigned to the group. On the other hand, we never hold overhealing against anyone unless they've got mana problems, and even then it's a question of how we can help them rather than displeasure or the like. While healing meters can be a great way for individuals to see what other people are doing and adjust where they are and the strategies they use, our ultimate tests are 1) "Did the boss die?" and 2) "Did the raid die?" If we down a boss with minimal deaths (minimal is usually dependent on whether something's progression or farm content) then everyone is probably doing their jobs and we're happy.

I don't generally PUG and I try to avoid it for raids whenever possible. I don't trust total strangers to stick to healing assignments or heal effectively unless I can see them doing it first. That's probably a lot of what you're running in to when you're in the PUGs.

On a happier note, they're actually making Beacon of Light count overhealing now, so this should at least be generally less frustrating for you. It's almost enough to get me to try healing on my paladin again. Almost. But having *my* holy lights stomped all of the time got me moved from the main tank to group because I wasn't performing well, which then tanked my performance farther because all I could do was play whack-a-mole against smart heals, which made it tank even more... I'm much happier on the shaman now. (Our healing lead also has a much better idea of paladin healing mechanics now and is much less likely to stick a pally on group healing, but I still like the shaman more.)