While it might sound strange for FBReader to go with a less common ebook DRM, there is a certain logic to the move.
Marlin is a consortium of tech companies that develops new DRM standards for license. I last mentioned them in 2011 when they announced that they had formulated an ebook DRM standard. No single company controls the standard, which means that unlike Adobe no one is trying to collect transaction fees and certification fees.
Adding ExpressPlay DRM won't help consumers read their B&N, Kobo, or Google purchased ebooks in FBReader, but it saves FBReader the expense of paying huge fees to Adobe while at the same time giving FBReader an opportunity to license their apps to ebook retailers.
Yes, Adobe is the most recognized name when it comes to Ebook DRM, but they're not the only supplier. Kobo uses a non-Adobe DRM internally, and so does the ebook subscription service Oyster and the recently launched middle eastern ebook retailer Kotobi (which is confirmed in an unrelated press release to also be using ExpressPlay). And I would bet that there are others which I have not found.
FBReader is adding ExpressPlay DRM because they want to license their reading apps to ebook retailers. I cannot predict whether they will have any success, but I can see how FBReader would be an appealing option.
FBReader has apps for multiple platforms, including Android, iOS (coming soon), Windows, and OSX. It's been around for close to a decade, and as anyone who uses it can tell you it has received numerous and regular updates. According to the press release, FBReader boasts five million active users and nine million downloads.
The apps currently support a number of DRM-free formats, including Epub, PDF, FB2, Mobi, html, DOC, and rtf. It is highly customizable, allowing readers to choose colors, fonts, page turning animations, dictionaries, bookmarks, etc., to make reading a convenient and completely customizable experience.
Update: I have been told that the new DRM support will cover Epub, and that all of the existing features will be available.
It's not clear how many of those formats or features will work with the new DRM version of FBReader after it has been licensed to ebook retailers, but I think it is safe to say that a serious contender just joined the reading app market.