I prefer reading Mobi over ePub on my reader also, but since I convert everything in Calibre and keep copies in both Mobi and ePub (just in case), the ease of DRM removal is my main reason for purchasing Mobi whenever possible. Not that it's difficult to remove the DRM from ePUB, but with the Calibre plugin, deDRMing Mobi is completely effortless (gotta love Calibre
).