If you have listened to them and don't like them, that's one thing. I think you are speaking from a lack of knowledge. I defy anyone to listen to their music and not like them.

I've listened to quite a bit of the Dead and Garcia - granted, much of music tastes are subjective, but I've never found their music skillful or impressive or interesting. To be honest, it bores me. Same with the Beatles.

Sometimes music can be impressive but still uninspiring - for instance, Beethoven was certainly skillful - but Franz Liszt had it all, and created some of the most adventurous music of all time.

I never criticize someone for their music tastes (except if someone likes Bon Jovi, haha) - but I often find that music tastes (and art tastes in general) reflect one's sense of life.