Am I the only one here that believe everybody is missing the main point? I mean: illegal sellers don't care a shit about selling MAME. They sell because they give you ROMs with an emulator capable of running them, for people too lazy to google them by themselves. I've never seen in my life anyone selling "the MAME emulator, no ROM included". They all sell "thousands of games with MAME emulator". They can use MAME, they can use RAINE, they can use anything to make money. If MAMEdev could hire an army of lawyers to chase down any copyright infringement, I bet ROM sellers would simply start selling ROMS with [insert any emulator here], no matter what license this emulator has. The difference between having a commercial license for the emulator or not only means that ROM sellers will break only 29436 licenses (the number of games emulated in MAME .150) instead of 29437. Do you REALLY believe they will give a single fuck about the MAME license??
Instead, there are thousands of open source software, released under commercial-compliant license, that goes on being free since the first day. Nobody ever thought about things like "I'll use your code to make the payable version of your same free software" because it's nonsense. You should make significant changes and improvements to the code to make it worth, but there are already so many flavors of MAME around that I cannot imagine how they would possibly do this...
People buying a MAME bundle don't buy it because of MAME, they buy it because of the ROMs. A change in MAME license will never change this. And I don't see any chance of a "paid MAME". Because it is not in dev's plans, but even because as the OpenOffice/Libreoffice history teaches us, should the dev divide over the license terms, nobody can prevent the "dissidents" to take the (eg) 0.150 mame source and produce the TFAETWNBACU (Truly Free Arcade Emulator That Will Never Be Allowed Commercial Use) and improve and distribute it with their preferred license...