OK, recognize this situation? You hear/see a good song on radio/TV, go out and buy the entire CD and then discover that there are maybe one or two good songs (one of which being the song you heard), but the rest pretty much sucks?
Maybe the rest of the CD even sounds different from the song you heard on MTV? Or the songs just aren't as memorable as the single?
If so, you've just been exposed to a typical phenomenon in commerical music.
So why doesn't this affect underground music (or independent music) in the same way? Simply because underground music doesn't sell through exposure in radio or TV. People buy underground music because a friend recommends a certain CD, or because they've read about it in a magazine or fanzine they trust. Or maybe they downloaded some MP3s from Napster or an FTP site and realized that there was more than just one good song on the album.
If there is only one good song on an album, people will know about it when they get to hear about it... Because underground music is exposed in a perspective - the entire album is rated and reviewed, and people aren't left to judge from just one song that someone else chose.
With lots of money the record labels can buy lots of exposure for the artists and simply fool people into thinking that the music they hear ten times a day is actually good. You can sell anything through heavy marketing, but without marketing, only quality stuff will survive.
In the long run, internet and MP3s (and future similar technologies) will probably make it harder for commercial artists to survive by releasing weak albums with heavy single-promotion - simply because most people will download the songs they care about from the 'net and ignore the rest of the album.