I may need to be able to read the book on several devices, which may not support the particular DRM.
I re-read. If the bookseller stops support for the DRM, I might not be able to re-download the book later if I buy a new device.
The application on my device that displays the DRM'd file, may not support my needs/wants for a particular font or display setting.
Applications and file format support for DRM'd files are, to say the least, very varied on different devices.