Yeah, this whole attitude annoys the snots out of me. I remember somebody bitching in the comments section of an NY Times article, about how adults participating in halloween indicated the infantilization of society. I politely tore them a new one, saying that while it's okay for kids to play dress up and pretend (in other words, use their freaking imaginations) just about any time they want, we adults tend to live very restricted lives. At work, we often have to abide by strict uniform or dress code rules, and there's a ton of rules, both written and not, governing what we may or may not say. So perhaps we need the outlet of dressing up more than the kiddies do- a chance to step away from our workaday "professional" personas, and try something different for a night. Also, to some of us, our costumes are more than just silly party outfits- Some of us choose to express genuine feelings or ideas through our costumes.
I really don't like the idea that somehow, if I go to the casino and chuck a bunch of money away gambing, or go to bars on a regular basis and drink like a fish, that's a mature way to spend my money or have fun. But if I spend the money building a beautiful costume to wear, that's just "so awful and immature" in some people's eyes.