Here's another one that might qualify:

"gay" - used to mean happy, carefree, etc. Now it means "homosexual" or "lesbian". What's wrong with just saying "homosexual" or "lesbian"? Well, "gay" sounds kinder and gentler, I guess...

I don't know to whose advantage this one might be, but it's odd how these things go in and out of style. The common insult in schools now, on the part of homophobic young fools, is to call someone else "gay"! The usual typical nonsense repeating itself yet again. The kinder, gentler word did not solve the problem, it seems.