If the conversion from printed book to ebook is 100% perfect, then I wouldn't even bother with the DRM. But it's far from perfect. OCR sucks. Typo here, typo there. Format oddities over here and there.
Just read Amazon customer's reviews complaining about typo and format oddities. They are everywhere.
As a side note, here's real life examples how DRM screwed consumers
http://en.wikipedia.org/wiki/Digital...t#Obsolescence.