I don't think it makes you personally a bad dj, thats not for me to say, but i think it would make me question a persons relationship to the music.
A person brings out a 3 track ep and you like 2 instantly and you can't give them the benefit of the doubt on the 3rd track, that you might grow to like it or find something in it? Think of your favorite albums and how they play out.
"Oh, this track wouldn't work on the dancefloor, i won't buy it" etc. Its just weird to think that tracks have to fit into this criteria to be worth buying or not buying, or essentially, worth listening to or not worth listening to.
Its like djing off a Hits of Chicago House record, instead of trying to check to see what the b-sides of those tracks might be. I dunno, i expect djs to actually like music, not to just see tracks as objects that they can bend and shape into a 1hr set.
I totally agree with Atheory and have thought about it in the past. I think a lot of it ties in with listening practices and generally how much time people are willing to dedicate to listening to music.
You hear people complaining that they don't have the time when really it's just that they have other priorities, which is fine especially if you have kids or something but like it or not this does affect peoples tastes.
Some people are just not willing to dedicate the time to listen to full tracks or albums not just once but multiple times. In that case why would you want to buy or be able to appreciate vinyl? Digital tracks or picking your fav out of the four track ep is a quick fix that saves time (and apparently money). It's perfect if you want some tracks to listen to in the car driving to and from work. There are A LOT of DJs like this and disposable music fits their needs perfectly.
This dedication of time specifically put aside for LISTENING purposes sticks out like a sore thumb whenever you hear sets or mixes from this type of particular DJ.
Of course this is only my opinion based on my personal observations so it isn't an attack on anybody in particular.