The FBReader code base has been scrubbed so that it can be "dual licenced", allowing a 2nd closed source version that includes support for one or more proprietary format (e.g. with DRM). However, Nikolay Pultsin indicates
here that so far he has seen no interest from the owner's of proprietary formats in a FBReader version.
Many e-book providers are lacking either a Linux or a Java solution (FBReaderJ is under development). So a closed source FBReader version would seem to make commercial sense as an inexpensive way to expand the market for a given format.
FBReader has rudimentary EPUB support, and this will improve over time. Adobe has the biggest stake in EPUB, and they would benefit from making their DRM "standard" for EPUB. So a FBReader version with Adobe DRMed EPUB would go a long way to pushing this new format.
MobiPocket supports many device types, but not Linux (except when it is tied to specific hardware, e.g. the iLiad) or OS/X. FBReader already has very good support for DRM-free MOBI files, and adding DRM support would help solidify the leadership position of this format (despite its significant technical shortfalls).
To make it a stronger competitor to MobiPocket, eReader needs to be available on the widest range of devices and FBReader would quickly and inexpensively add new device support. Both eReader and EBookWise are owned by FictionWise, so perhaps FBReader can also provide an upgrade path for EBookWise. However, I'm not sure how closely the IMP format is tied to HVGA resolution and how this might effect portability.