Look, if you sign up, you sign up to actually give marrow if there's a match. Period. Unless you're not healthy enough, there's no excuse for welching on a deal that involves someone's life.
I agree you can't force them to go through with it, but the effort needed versus the outcome if successful are not even comparable.
This is why Flynn vs. Holder is so damn important.
If the goal is to actually save human lives, more people need to choose to let doctors invade their bodies to take the cells they need, take the time off of work, away from their goals, etc. The only way that most people could do this for a stranger is to be compensated. I predict great things to come!
http://marrow.org/Registry_Members/Donation/Steps_of_Donatio...
Up until today, it was illegal to seal the deal (i.e., pay your donor). Now that people actually have a rational financial reason to trade a little bit of their own lives to strangers (needlesticks aren't painless, and donors need to sit for hours to actually donate the cells), they'll go through with it a lot more. The donors win, and the recipients win. That's a great thing!