As said, the third deck is just a tool. Even if you mix your records normally (meaning you beatmatch and mix one tune at a time), the mix will still sound better, because you can put your next record on before fading the track that is still playing out, which speeds up the process of mixing, 'cause you can look for your next record while listening to the mix on headphones, adjusting the pitch if necessary...

But I must agree that I've also seen many DJs fake it, leaving the third deck spinning. Why they do it, I don't know, you can't get a good rep by faking; if you don't have the skills, why bother faking it?