The math teacher in me has to point this out: the Disboards probably isn't the best audience to sample. Those once-in-a-lifers are probably a much smaller percentage of Disers than those who go fairly frequently.
As for us, here's our history:
2008: I had gone back to work, so we got to take the kids to WDW. I was also looking at a mastectomy a month after our return, so it was a great distraction.
2009: Disney offered the 4/3 deal after the economy tanked. I had just finished radiation and reconstruction, and was lookint at a thyroidectomy that September (yeah, it was a rough time!) and the distraction was great.
2012: After 3 years, we figured we were due for a return.
2014: it will be our 25th anniversary, our son's 16th birthday, my older daughter's middle school graduation and our younger daughter's elementary school graduation. Sounds like an excuse to me!!
So I voted for "every 3 years", but there's no real pattern here.