I've got to disagree with you here. DJs are often Jeckyl and Hyde nutters who can do one stunning spot and one not very good one; with regard to the dancefloor and general buzz.
As a promoter you want to cater to the crowd and put on DJs you think will entertain them and play some great things they don't hear very often in the right context (ie mixed in with the bigger records). Most DJs have very large collections and can do newies, oldies, modern, R&B, cha cha cha on any given night. Hopefully they will play whatever the crowd responds best to and throw in the odd curve ball occasionally that fits into the set. Many DJs drift into self indulgence, myself included, because they have heard their records 100 times but some of the crowd may not have heard them once. It's a tough balancing act and a promoter will try and get the right balance for the event but if the DJ doesn't deliver on the night its not the promoter's fault. He will probably drop the DJ for a while but then may hear him DJing elsewhere when he plays a blinder. It's far from an exact science and there aren't enough DJs of such a high standard that they will always play great sets. If they did then they'd get criticised by some for being too safe, or going home early, or taking or not taking gear. Speaking of which booze and gear are factors a promoter can't always bank on with his DJs and they often affect a set.
Ady