The only thing that makes me question the 1%-2% figure is that Titanium Backup, ROM Manager, and SetCPU are the #3, 4, and 9 top paid apps in the market
Well, most of the time when you flash it downloads your apps from the market. So I would guess all those apps have at least 10X the number of downloads they wouldn't otherwise have. I'd think updates also impacts that number. I don't know if Google filters that by unique users or actual downloads.
I mean, Titanium Backup and Set CPU are/were almost required apps for rooters, so even 2% would be @ 1M users.