Wow, I guess musical busniness sense is really lacking on this board. When a label releases an instrumental tune and then re-releases a vocal mix some months/years later, it's basically to drive for more sales. You guys may be loyal fans and clubbers, but to others this is a business.
Anything that gets picked for radio play usually tends to make a label a lot more money. Ideally, more exposure/publicity = more money for the artist and label. The more mainstream market tends to actually BUYYYY records and singles as opposed to downloading them on their favorite file sharing programs. So when a tune gets hammered on the radio, it's a good thing.