I do think double melee comps are bad for the game and there's an objective reason for that. They're incredibly limited at the very core - it's all about both melees connecting damage on a single target long enough to drop it. There's simply not much else to do, and it's very unforgiving for both sides. They can win because the enemy team did a single kiting mistake, just as they can lose if they did a single mistake with their cooldowns or anti-kiting/gap-closing tools. This is by design, because melee is supposed to do more damage as they're primarily limited by their uptime on the target. When the amount of "key" decisions throughout an entire match that shape up into a victory or a defeat shrinks down to one, it is bad design by any standard. Wizard or caster/melee/healer comps don't have this.
Don't get me wrong, elemental shaman 100 to 0 kills are also a bad feature of the game, and so are hunter shenanigans. But those are balance issues, I am talking about overall design in this post.
Just in case, I've only played warlock since mid cataclysm. Priest, DK and enhancement shaman before that. I've played as much double melee and melee/caster/healer as I've played wizard comps.