Tracie eloquently describes one of the top reasons I love nail polish:
[Nail polish] might just be the only form of primping and grooming that isn't rooted in making oneself more appealing to men or exploiting women's insecurities. It transcends skin color and hair texture and face symmetry and body type. (Nobody ever says, "I look fat in these nails.") It's an aspect of beautification that circumvents any conspiracy theories of the patriarchy's hegemony because the patriarchy simply does not give a shit.
I tend to feel more comfortable about the relationship between women and makeup versus women and fashion. Makeup is an easy way for a woman to feel good about herself and celebrate her own face. Fashion can veer off into territories that can bring down self-esteem; certain fashions just aren't for every body type, or for every budget, or for every weight.
But makeup isn't without its shadowside, either. For example, too much makeup can send a negative message... and yet the concept of what is and isn't "too much" is, generally speaking, very subjective. And then there's the fact that society expects women, particularly professional women, to wear at least some — and if you don't, that sends a message too. I think we've all had days where we head off to work, or out and about, and opt to skip makeup for once, and co-workers/friends say, "Oh ____! Are you feeling okay? You look a little sick."
Nope! This is just my face sans war paint! Mmm, awkward...
Nail polish is an element of beauty and grooming that, like Tracie said, transcends body type, and it's free from the Cosmo-esque "how to look sexy tonight" directive.
Nail polish can be cheap and easy, or it can be indulgent and detailed. You wear it, not the other way around. It can be whatever you want it to be — so you can do whatever you want to do.