" think of the possibilities"
What might those be? The source has been available for years and if anybody around here needed a hardware reference (and in the case of some systems with a handful of games then occasionally even a software reference/ROM map). I guess it could be copied into the wiki or something more easily. With the source being available then if someone had wanted they could have added better debug options than might have existed in stock.
I think the main people to benefit here are MAME cabinet builders, those doing some of the handhelds (GP32, pandora, dingoo...) that want to have a kind of repo and emulator authors that might want to borrow code or use certain libraries. All kinds of people I like and have had great conversations with over the years but the MAME licenses hardly held back ROM hacking.