Its not gamebreaking in any way, you can spot a healer without it just as easy, put class colors on nameplates and there you go, just find which of those is the healer, also you can notice the healing effects, usually green stuff for druids or penance/a priest not in shadow form.
Also, this is a large misconception between healers, that they should be able to keep themselves alive alone against 2, 3, 5 enemies. They should work as a team, and it's the DPS job to look after their healers, to save them, to keep them from dying. In addition to HHTD, an add-on, My Healbot is dying should be implemented, so that DPS can save them with any kind of CC, slow or interrupt. That's teamwork.
Also, HHTD shows who is healing your side, so its easier to save your own healers too. While not wasting time looking for them. I started using HHTD just a while ago, and it just makes me find their healer faster, as I could do it myself already.
This is my main, and healing is what I love the most. And tell ya, the add-on isn't the issue. As a long time alliance player in PvP I alwas noted that even back in the days horde players (usually more skilled) would always come for me as soon I lay a heal, sometimes they would even pass by the front lines and chase me. It have always been like this and will continue EVEN if blizzard ban this add-on, cause the only thing the add-on does is make your reaction faster, but skilled player will still continue to hunt down healers as soon as they enter in the battlefield.
Another misconception that happened in cataclysm is that most healers got used to Nuke heals in LK, so with good gear, you could easily stand in a single position like a turret and overheal all the incoming damage using CDs. In cataclysm this is not the case anymore. Most of the classes got some kind of silence, interrupt and CC to annoy healers. But also, with the size of health pool, bursts have become less of a problem, so you don't need to keep healing someone all the time. You have more time to not cast, you have time to run and use your instant while you run. If someone go after you, don't try to outheal the damage, you probably gonna get interrupt/silenced. You need to run, use instants, LoS and most important of all, trust your team mates to save you. Running will keep you alive long enough for them to do it. yes, is hard to find such skilled people to save you in pugs, but with time, players will get used to that. In meanwhile I suggest to always bring a friend with you, and keep together.
I use this add-on with my DPS alt, and always go for healer, but healers that have good mates usually get me. Healing can be very frustrating in PvP, I sometimes go nerdrage, you know keyboard smashing by cheer of stupidty from other players, but that's PvP, get used to it. I suggest using this add-on too.