Desmume has no side cotributions due to using SVN and Sourceforge.people, are you really know what are you talking about ?
yabause - afaik there was no side contribution, in fact project seems almost dead for a years, even it OS and far from finished and have really bad (compared to SSF, closed source, made by single person) compatibility.
retroarch - made by very few people, not sure there was anyone's other code in the core.
MAME - VERY HUGE project, there quite many pull requests, but from people which is lets say "almost" the MAMEdev and close to team. but almost ZERO contributions from really side persons.
Final Burn - no side contributions afaik. (except from several from a guy who port it to Xbox)
Desmume - no side contributions
GlideN64 - this project OS is more than a decade, but all this time was improved and maintained by SINGLE person (Gonetz).
NullDC - no side contributions at all.
reicast - there was few interface/backend/portability side code. but no any emulation related. with iirc one exception - DC microphone emulation
lxdream - no contributions. project dead for a long time.
bsnes - no contributions. all was made by byuu only.
and I can continue and continue and continue this list of open source emulators to very very huge size.
but results will be the same - there no any benefits for emulation project going open source.
with a very few exceptions like Dolphin is.