I recently started putting in the lottery pool at work. Even though I know that even with the 20-30 tickets our combined money buys the odds of hitting the jackpot in my lifetime are infenitesimally small. And probably a case could be made that a lottery pool is actually worse than playing alone since it doesn't significantly improve your chances of winning, but does significantly decrease your share of the jackpot if you do win.
I have no expectation of winning. I do have what I believe is a perfectly rational reason for playing though. Instead of greed, it's fear. I don't want to be the poor schlob that get's interviewed by the TV station the day after our group wins...
"Sooo, betcha feel stupid now for not playing, right?" 