I find myself in the minority here. In other words, I prefer single-typed Pokémon. But honestly, my preference for dual-types largely depends on the types you mash together. Moreover, I just personally think single-types are awesome, mainly because I like more of these Pokémon than the rest.
To prove my point: aesthetically speaking, I hate how some Pokémon look as if they were forced to appear more equally for their dual-types. There are a lot of well-designed ones, I give you that, but some just stink. It just starts to seem as if creativity isn't limitless, if only momentarily.
Sometimes, the duality doesn't even make sense. I don't really feel like nitpicking but I honestly think a few Pokémon turn weak in battles just because they had another type. Not to mention how it contributes to how forced the appearance of a Pokémon looks.
There are positives to dual-typing, most of them mentioned by you guys. I particularly like the gameplay positives of doubling STAB that comes with it. And it sort of balances out the negative ones, which makes me happy. In effect, I hope the next Pokémon generations would have better examples of the dual-types done right, and make me think otherwise.