You're misunderstanding, theburner, that's all.
Music is not the END ALL decision in a relationship. You stated, "She's great, a doctor, loves kids, but listens to Milli Vanilli"... that's a perfectly valid exception. I am stating an example that I would use, with the whole "mass marketed MTV bands" thing.
I don't know how old you are, but you must live under a rock if you think only 15 year olds are like that. I've met a LOT of females my age (24) and older who are like that, and while they're good friends, a lasting relationship with them would be pretty dull and uninteresting. Yeah, they may be good people and all, but I want more from a relationship than, "Hey, we get along, let's spend the rest of our lives together!"
Why? Well, in this one case (which, again, is just an example of how music can matter), I find that people who enjoy that type of music aren't too keen or aware about the world around em. Their sense of humor is somewhat basic, and they are often unable to hold semi-intelligent convo. Their likes and dislikes are based off of what their friends/TV tells them to like. Trust me, I've met these people before all over, and while they aren't ALL like that, most of them are. So, it's fair to assume that with someone who enjoys that type of music, chances are very slim to none that they'll turn out as an ideal partner. That's what I'm trying to explain.. and that's not being shallow.
Shallow is saying something like, "I won't ever date fat girls" or "Redheaded people are annoying, so no, I won't date one," not, "I've found that 8/10 people I know who like this type of music generally aren't compatible with my other interests in life. While I would like to be friends with them, I don't think a relationship would work out." Big difference!
What I am saying is, you can get a general idea of someone's personality by their tastes in music, that's all. I hate country music and sappy romance flicks. If I met a girl that LOVED country music and her fav movie was "Maid In Manhattan", it's not like I'll turn my back on her and tell her to fuck off, but it's like anything else... talking to them, spending time with them, etc... you can get a good idea of a part of them by those tastes. I'll still talk to em and see how they are as a person in general, but, will she enjoy my dead baby jokes? Will she enjoy Night of the Living Dead with me? Probably not. Will she want to go to a Mike Patton concert with me and enjoy it with me? Probably not.
Everyone has different interests that they want from relationships. That's not being shallow.
Like I said, I challenge you to try it. I didn't say 100% of the time people who like Britney Spears and Hoobastank, for example, will be uninteresting drones, but... it's pretty high up there.
Getting a general idea of someone's personality, likes, and dislikes because of their tastes in music and movies is NOT being shallow. It's no different than if you got to know them and later found out you weren't all that interested in em. Is that shallow? No. I can understand why you'd think it would, but just take a second out to understand what I'm trying to say. Don't jump to conclusions
To answer the topic's question: how much does it matter? To me, a good amount. I can walk into a place wearing a Mr. Bungle shirt, and if a girl comes up to me and says, "Hi, I really love your shirt!" I can know then and there that the chances of us being compatible with additional interests and sense of humor will be quite high.. that's all.