In your opinion it doesn't work, for plenty of others it does work. If it is danceable it will work, Stafford last week showed you can play the genres and make it work, dancefloor rammed all night. I agree with floaty stuff (what genre you referring to though?), that is no good at an all-nighter, you need dancers although the odd slowie when programmed right can and does work. The venues that cater for the down tempo stuff is a different matter....don't go if you don't like what you hear. Not every venue is put on to satisfy you, me or anyone else but there are venues out there that cater for the taste of each of us.
As for the change in styles that depends on how good your DJ is. There are DJ's who can change the style and still make the set flow and work for the dancers.