Most people prefer to keep their books, particularly if the books are a source of constant reference. I like to keep mine, and I suspect my preference is in the majority. In your own case, DRM would presumably be unlikely to cause you any real grief if you're never likely to re-read the books or even keep them.
However, keep in mind that DRM would cause you some grief when it came to your choice of device to read on. Many, many people use MobiDeDrm precisely because it affords the opportunity to 'free' a legal purchase from constraints, so they can then read it in the way they choose.