The beauty of this system is that it won't lose any comments. Even the people who are forced to enter a captcha (which usually decreases the amount of comments) would rather do that than know they wrote all that for nothing.
As the author says - a CAPTCHA solution reduces comments significantly, lets say by 50%. So if Mollom marks 10% as spam, and half of those are actually legit, 2.5% of comments are lost.
But that's not a bad price to pay for getting an hour of your life back a day.
I'm using Akismet on my blog, and comments considered spam are held in limbo where I can approve or delete them. No comments should be lost unless I accidentally misjudge a flagged comment.