Just by having a healer a premade team has an unfair advantage. Healers are a resource that both or neither sides should posess on the same basis. If I PuG whether or not I have a healer is a matter of luck, an RNG roll. Premade teams have a healer by a matter of course and fact. If my team's healing capacity is dependant on a random number, why should the enemy team have a 100% chance of having a healer. I'm ok with an RNG number. But then have the enemy team composition depend on luck as well. This can decide matches even before they began.
Seeing as you have plenty of replies already, I figured I would take this little nugget out as it has nothing to do with premades. What will you do in your solo queue when the other side has a healer and you do not? This is the exact philosophy of every person advocating for a solo queue. They have a laundry list of things they feel to be issues that will all be resolved if no one can group.
If you really need a healer that bad, then maybe you should take on that job yourself. 100% chance to have a healer on your team. The biggest problem with the solo queue mindset is that you only care about yourself since you do not know anyone else in the warzone. Most people don't want to heal in PVP without knowing they have support; tank classes that taunt and guard, DPS that can focus fire. What you are asking for in your statement here is to have good team composition to make the games more competitive. Easiest way to do that: group with your friends. What a surprise.