The simple answer to your question is there are a lot of good DJs out there who play off original vinyl, and fill dancefloors, and try and introduce new records (Because let's face it, if DJs didn't try and introduce new records we would still be listening to the same 23 records that were played in the mythical first ever Northern Soul spot)
The point you're asking about is quite simple. A DJ who plays to an empty floor, and continues to play to an empty floor just isn't a very good DJ. (Unless of course it's the sort of venue where the DJ is expected to play new and unknown things and not expected to fill the dancefloor (They do exist))
I always play off original vinyl, and always remember that it IS a scene where people come to dance. But I always like to try 'new' records out in my set. Sometimes they work, other times they bomb completely. That doesn't mean it's a bad record, just that other people don't share my enthusiasm for it. But if I clear the floor with a record, it's pretty much a certainty that I'll get it full again simply by playing a record I know people will dance to.
The other thing that I think people forget is I've been DJing for a long time now, and there are some records that I am simply bored to death of hearing, and playing. I usually sell them when I reach the point of boredom, so that I can't play them again, Again, that doesn't make them a bad record, just one I don't want to hear again for a long time (And I can't understand why people want to dance to the same records for years and years either. Don't you get bored ?)