I have to say I'm pretty unconcerned about a license allowing commercial use of MAME/MESS. Keep in mind that most source-available projects allow commercial use. Linux can and is sold in a variety of projects. I've personally worked on scanners running a modified Linux kernel. And I can understand how switching to a standard license rather than the current customized license will make it easier to share code between projects.
I do, however, see how it could be a nightmare identifying contributors to every file and seeking permission to relicense them individually. Even if it could be done accurately, what sort of program would you actually end up with if you wanted to use only code under the new license?
I'm curious about the rules regarding projects with many contributors switching licenses. Wikipedia switched from the Free Documentation License to Creative Commons BY-SA. While they did take a vote, they certainly didn't require permission from every single contributor.