Like minds, they work better when open - where's my guru? (Cliche, but it makes a darn groovy poster, baby!)
They are groovy for blocking the sun from my skin when I'm at the beach. (Which is rare-- why should I subject myself to the public humiliation called "swimsuit"?)
Only wussy people use umbrellas when it's raining! If it's just sprinkle-ing, it's not going to mess up your hair. (And why do you care about your hair so much in the first place? If you think that you're going to get styling products all over your skin and clothing, you're obviously wearing too much!) If it's pouring cats and dogs, a stupid little umbrella is not going to protect you! (It's going to flip inside-out and the little metal arm-thingys are gonna poke you in the eye! And as my friend always says: It's all fun and games until someone loses an eye; then it's just fun! ...wait, that doesn't help my argument, does it?)
Maybe I'm just immune to rain, growing up in the Northwest and all. (We had about 100 straight days of rain last year.) Maybe you aren't a wuss... But I bet you didn't have the fun of making up a rain danse when you were in elementarty school!