"(thing with fans) vs. (competing thing with fans)" often results in fans of each thing angrily debating why their favorite thing is awesome, everything else sucks, and everyone who disagrees with them should be described with a variety of offensive slurs.
A lot of the time, though, the people who complain about versus threads are not exercising any discrimination between flamewars waiting to happen and harmlessly fun versus threads. They just see "versus" and have a knee-jerk reaction.
Examples of flame-inspiring versus threads: "Microsoft vs. Apple" or "Gordon Freeman vs. Master Chief"
Now here's an example of a thread that I think is harmless: http://www.escapistmagazine.com/forums/read/18.180368-Poll-WALL-E-VS-R2-D2
If you look at the comments, there is no flaming over the choices. It is just a harmless, fun scenario of imagining the scenario of a bunch of fictional robots fighting. Actually, the least constructive conversation in the thread is various paraphrasing of "We don't like versus threads."