If you buy DRM-protected books from B&N, then IMHO it is better to buy DRM-protected EPUB files, because you can remove DRM protection from B&N (Barnes & Noble) EPUB ebooks.

You can also buy EPUB (or PDF, but EPUB is better) books in US stores that provide Adobe EPUB ebooks. For example: Books on Board, FictionWise. DRM protected EPUB ebooks works on Sony Readers, so you don't need to remove DRM protection from EPUB ebooks.

However the situation with audio and video content shows, that some stores leave the market and some computers crash, so users are left with legally purchased DRM protected content that they can no longer use. Also your next ebook reader may not support EPUB or PDB, and it is always easier to convert DRM-free content.

So I highly recommend to buy DRM-free content or at least to remove DRM protection from any ebooks or media files immediately after purchasing them. But please do not abuse rental licenses!