People who buy books from multiple sources, not just from Kobo, also use ePubs--either with Adobe DRM or DRM free. My preference is to purchase from Kobo because I read on multiple devices and I really, really like sync. It seems to me that over time the prices at Kobo are pretty much the same as the prices from other book stores. Sometimes you can get a less expensive version from a a place like Amazon, or BOB, and if the difference is more than a dollar or two, I will buy there and read an ePUB. I think price equality may be different in some countries, but for Canadians, this seems true. The implementation of the DOJ lawsuit ruling could turn this upside down, as it is US only (hummm, that family summer house in upstate NY could come in handy)
I check prices using calibre, btw.
Although I prefer to read Kobo ePUBs, I always make a drm free backup for all of the reasons Peter gave. My husband and son read ePUBS so they don't screw up my sync. I read on multiple devices, as stated earlier and they do not.
There is a group of people who never buy books at all. Some of them use Overdirve to borrow library books, and they go onto the Kobo eInk devices as ADE DRM'd ePUBs. Some people get every book they have from torrent sites or other places where books under copyright are made available in ways that may violate the law. I am not giving any kind of opinion about this, just explaining that the Kobo devices are used for ePUBs (or PDFs) if people are getting books in this way.