The problem is that most premades are cowardly and it's easy to come to the realization that if your guys aren't dying, you probably won't lose. Now, if you've 3 (or more) healers you're not going to have much DPS either, which means if the other side has any healing they also enjoy the same 'can't lose' advantage you have. And sure strategy matters but now a game of Alderaan would be like 'they got the first tick, so thanks for coming and DPS pretend you're trying to kill someone for the next 20 minutes." Is that really the kind of gameplay you want to encourage? Do you really want 3 DPS glue themselves to a node on Voidstar/Novare Coast and just cap nonstop from begining to end? It's a perfectly valid strategy in a heavy heal versus heavy heal situation, and you can even say good players will be better at chain capping (they presumably will know when is the best opportunity to do so and throw out CCs as necessary). Sure this is still a matter of skill, but it just feels weird to say our DPS was better at clicking the node than the other side which is why we won. Shouldn't we won because our DPS is actually better at killing their guys than their DPS?
I actually almost never use pre-mades. For the time being, I am in a PVE guild (no one else even has a full set of partisan let alone conqueror). I play against pub/imp premades. Sometimes my pug wins, sometimes my pug loses. Yesterday I was in a zone where the highest hps on my team were 25k, I currently have 30k as a sorc. It didnt matter that I was the top healer in the zone, it didnt matter that the other team had no healers, my team simply could do nothing to them.
And from my perspective, the team with the best dps is more likely to win than the team with the best healer.
If you have 6 dps that are better than my 6 dps, it likely wont matter if my 2 healers are better than your 2 healers. The reason is, you are outputting way more damage way quicker, so your healers (even though they are worse) can keep up with bad dps, where even the best healer cant outpace focused top end dps.
If anything good dps + good gear, is really the ticket to winning. But many dps are not running full expertise and then wondering why they cant kill the healer who has more hp and more expertise.