the other annoying problem, when mods DO start using common "dynamic" libraries, is that Mod_A will be updated and depend on Mod_Library v1.2, but Mod_B's author will be doing exams or something, and require Mod_Library v1.1
I just downloaded a mod for my kid that required 3 other library mods. All code, all tiny, statically linking that content would make this download a lot more convenient now, and a lot less prone to versioning conflicts later.
Happens either way - remember when BuildCraft 6.1.5 was forced to break its API to restore an important feature and it broke Binnie's Mods for a few months, requiring me to create the world's hackiest workaround for FTB just so I didn't break Binnie's license?
Or when RF's API had a breaking change in 1.7.10, causing quite a few mods to act weird for a while.