and dubstep IS crap. if i didn't believe that to be the truth i wouldn't hold it to be my opinion. i can assert said opinion if i want, just as you can assert yours. besides, it's pretty obvious that when people say something is crap they are voicing an opinion, and it's unreasonable to expect everyone to write "in my opinion" before every statement they make.
I guess that depends on whether you're saying "I don't like/understand it" or "I think it's crap". The first is perfectly understandable - I don't particularly like or understand "urban" music, but at the same time, I can appreciate the musical quality and talent of some of the artists (for example Beyonce has an absolutely stunning voice).