If all we cared about was the message and the meaning behind the lyrics, then we'd just listen to poetry. However, I listen to music not only for the lyrics, but also the music itself, the talent behind it, the experimentation, the originality. While lyrics are important, they are not the only facet of the music. Thus, the reason we dislike many of the bands is in the music itself. I agree that many of these bands have a good message, but the music lacks originality and ingenuity. And honestly, bands like Linkin Park don't have a very good message, it's just maudlin, cheesy, "I'm not good at anything, everyone hates me" crap. And if they do have a good message, they often repeat it over and over again. People don't make music just to spread a message, they do it to express themselves, tell a story, and make something they view as beautiful and powerful. I'm not saying this just as a fan, but also as a musician.
Also, in your comments about hip hop, the majority of the popular music is terrible, but there are groups below the radar of radio and television that are incredibly talented, original, and provide a good message. Deltron 3030, Kool Keith, and Dalek to name a few. Dalek especially has beautiful lyrics; they are, in true meaning, poetry. Deltron is very intelligent, and often talks about how he tries to use rap as an art form while so many else out there use it for image and just imitate others instead of making original thought.
Finally, most people (at least the good ones) who listen to obscure music, do so because they like the music. Anyone that listens to an unknown group just becaue they're unknown is as big of a "poser" as someone who likes mainstream music just because it's popular. These groups have incredible talent, not to mention values, ingenuity and soul.
