This is a really interesting debate and I love that the answers are so varied.
For me, I prefer longer trips more spaced out for a lot of the reasons stated by other posters. Even if I knew that I would be coming back in just a matter of months, I would feel rushed if I were only there for 4-5 days. Plus, as crazy as it sounds, the anticipation is such a big part of the enjoyment for me. I love meticulously planning the trip, then reading Disboards and everything else I can get my hands on to prepare for the trip. Also, my fear is that if I went several times per year, Disney trips would become too routine. I know that's probably crazy to think, but it's what I've always been afraid of.