An interesting point, but the way most program works is that there's a minimum needed before the send a check, but it accumalates as long as is necessary. So, eventually, some years from now, perhaps, we'd get a check.
I do think you raise a very important point, though -- the amount of money that the Wikimedia Foundation would make from such programs is not likely to be huge, anyway.
By way of comparison, I conservatively estimate that if we put paid listings from Overture or Google into the search results page, we would make $1000-$2000 a month. I'd be very opposed to doing that.
--Jimbo