As a general rule, art reflects life. Our society has become so immune to swearing (as someone else said "inocolated) that it is now considered almost an acceptable part pf communication. I find very little merit in the hip hop music with lyrics that encourage killing and treat woman as no more than sex objects, but that reflects life, or at least life as it is in a very sad part of civilized culture.
I tend to agree with the concept that one who must result to vulgar lanquange to get a point across (in song or in life) is lacking in both verbal and social skills. I find it all too normal to hear people swear in front of woman and worse, children. This reflects no respect for others and little self respect for the person choosing to swear.
I don't recall who it was, but a well know person, known as an intellectual once said something to the point "I've never met a person, who used profanity, who interested me in conversation". Personally, I find people who routinely used profanity to be boring and given ehough time in conversation to reveal themselves to be rather ignorant.
In answer to the original question, I personnally do not think swear words belong in lyrics any more than I think something like a crucifix in urine is art - but - art reflects society and society has determined that vurgar words and images are acceptable.