Since it inherits from SPList and the only way I know to create a SPDocumentLibrary is to convert it from an SPList object using:
SPDocumentLibrary lib = (SPDocumentLibrary)list;
Why would anyone use an SPDocumentLibrary until the very last moment when they absolutely needed the member of that class?
It just seems like an extra unneeded step to convert to a SPDocumentLibrary unless you specifically need to access one of its members, most of which are already part of the SPList class. Maybe I am missing something, but I frequently see people doing this conversion, then never using any specific to the SPDocumentLibrary class