There already is a mupen64_nogui binary when you build from source, you know. I've tried, i always get a binary alongside with "_nogui" attached, and i never applied a patch. Weird.... That, or my memory is REALLY bad.
Anyways...
I don't see why we need something like a "libmupen64" but i also don't find it a bad idea. Maybe its in Mupen64's future somewhere... But then again, there is no real reason you can't just write a GUI into the source code.
I believe a GUI is important, not only for those who are new to PCs, but to those who are experts. GUIs can make debugging quicker or occasionally more useful. I think Mupen64s recursive ROM browsing feature is the greatest thing since sliced bread. Also, GUIs, a lot of times, can make work less tedious, just as a terminal does for other things. I'll tell you right now i would've destroyed my file systems multiple times if i did all of my partition work from the command line. I've done some from it, and I'm certain I couldn't do much more... Plus, Isn't configuring plugins so much better in GTK then say, a few dialog commands or a ncurses GUI could ever get, even though technically both are GUIs? Because, I know INI/file based options are nice, but they aren't very well/portable if you want to make it work in real time. Gconf? Yeah, its alright, but isn't that mainly for Linux, and isn't it tedious to edit Gconf keys on the command line anyways? Eh, I guess the point is, GUIs are and always will be important. Terminals aren't important all the time anymore, but they come in use a lot for those who know how to use them!