We (DF and I) are having our engagement party this Saturday and instead of gifts, we've made a money tree (mostly just out of a minature christmas tree though :P)
But our main reason for us to ask for it was because we don't wanna receive the same gift three times over.. And the fact that most of the people we've invited haven't got time to buy a gift as they're very hard workers, and that some of them have also just gotten back from a trip and just can't find the time to get a gift.
Another reason we chose a money tree is because my DF just found out where he's working next year, and that cash can go towards helping set up our future and be a little less to worry about for us in the not so distant future.
I dunno why, but I feel bad just asking for gifts unless its Christmas or my birthday.