I bought my Kobo primarily to read library books. Lately I've been getting 3/4 of the way through a borrowed book and have to return it. If I really like the book and there's another hold on it, I buy it off Kobo or Kindle (whichever is cheaper) anyway.
That said, if I felt the need to read Kobo books on my Kindle or vice versa, I'd convert and un-DRM stuff I've purchased, but not otherwise.