Because it's spreading misinformation that could prevent people from becoming marrow donors.
It might prevent people from becoming donors, sure, but it's not misinformation. You may think that the risks are small relative to the benefit provided, but you shouldn't pretend (or try to convince others) that there are none.
Take five of your minutes and give them to somebody that needs them.
It takes five minutes to register. It clearly takes more than that to actually donate. You're not doing anyone a service attempting to conceal this fact.
I think you're confused about a couple of things here.
1) I'm saying to take five minutes and register as a potential donor. If you're found as a match, you're not required to donate. You then get to chose if you want to donate more of your minutes.
2) You're vastly overstating the liklihood that you'd have to undergo surgery to donate bone marrow. This is typically referred to as FUD, and is generally considered a bad thing to do.
Because it's spreading misinformation that could prevent people from becoming marrow donors.
As a way of balancing this, here: http://marrow.org/Home.aspx
They'll send you a free kit. You swab your cheek, mail it back, and then you're in the registry.
Take five of your minutes and give them to somebody that needs them.