Words have a lifecycle; hey are born and they die. They die for many reasons; some die naturally (get out of fashion). Others die more untimely death (for example, they suddenly acquire a negative connotation). There are also words that get perceived as politically incorrect – they are on life support because there is still a large group of people who use them, if just to spite everyone else.
This is a good thing. Language is a strange combination of social conventions and explicit structure, and it's dynamic to reflect the changing needs and thoughts of the society. A language that wouldn't be allowed to change would be dead. I'm glad unfriend is now a word.
It's interesting to note that the half-life of phrases seems to have decreased over the past fifty years or so – we get tired of words more quickly, perhaps because as consumers, we're not satiated and are looking for the next big thing faster and faster. We react to words, and a new word can make us switch from one product to a different one. Inexpensive becomes economical and we suddenly stop thinking of ourselves as cheapos and instead pat ourselves on the back for not wasting money.
Branding is also one way to keep up with class inflation. We don't want to be gardeners, stewardesses or waitresses. We want to be landscapists, flight attendants, and servers. A different word – borrowed from what used to be a slightly different role – changes our perception enough to trick the same minds of ours that make us think that it's somehow less glamorous than it once was to have such a profession.
However, this can also be used as a political tool, to cover up issues or make them seem less like issues. What does vertically challenged mean? Is it because it makes us appear to do something about the issue? Or are we ashamed of the fact that the characteristic has found its way into popular culture as a signifier, just like retarded has?