DISCLAIMER: Purely my opinions. Micko is making all the binding decisions on this, not myself and not Aaron. As a Linux guy I know a large headache's worth about licenses, but I'm nowhere near in charge of these decisions.
1) Nobody on or near the team has the time, money, or lawyers to enforce the additional license clauses which make us non-OSI compliant. There was an actual formal call for this at one point, and everyone declined, including Haze. Furthermore, the additions were written by Aaron rather than a lawyer, and I think you don't need to be Admiral Ackbar to understand the trap possibilities there.
2) A large number of Bad Guys are making and selling commercial MAME ripoffs; they're in even big-name stores in Europe and Asia, there's the XXX-in-1 PCBs, there's the PS2 arcade discs Hamster made for Konami, there's Raiden Legacy, etc, etc. So it's not like the license is stopping anyone from doing unspeakable things.
3) The license has been a barrier to cooperation with other emulators. For those who haven't noticed, MAME has had it's clock thoroughly cleaned for fast and accurate emulation of modern and 3D systems by a parade of true open source emulators including SuperModel, PCSX2, Dolphin, DeSMuME, and nullDC. (Disclaimer: I'm involved with the SuperModel project, and I'm a fan of the rest of that list). Similarly, in many cases where other emulators have wanted to use MAME components, we've had to tell them "no", which doesn't exactly make them have warm feelings about the project.
On the MESS side, there's an even more formidable list of open source code which could be used to improve things if not for the license; our C64 audio emulation is terrible by any measure, and being able to drop in reSID like everyone else would be a large instant improvement.
Note also that nobody is selling any of those emulators in spite of their use of standard licenses which allow it; the GPL has proven legal teeth in both the US and EU, and it's hard to compete with free.
4) Given the first 3 points, in which the current license does nothing to bad guys while restricting only the good guys, the desire is for the license to a) represent actual legal reality and b) make things easier for the good guys.
There is no intent on anyone's part to start selling MAME or to commercialize it in any way. (We've seen the rise and abrupt failure of Ultracade, why would we want to go anywhere near that tire fire?) The nag screens will still tell you not to play ROMs you don't own, the readme will still say we don't like commercial uses, and we'll continue to tell ThinkGeek that we don't want them to sell MAME. If we end up in a license situation where we can offer official iOS and Android versions FOR FREE in the app stores, that would be amazing, but I'm not counting on it.
5) Nobody's forcing anyone to do anything, nor will they be forced in the future. We're currently identifying the contributors to each of the source files and having them tag acceptable licenses. Nobody knows the final outcome yet, and MAME will continue to be distributed under the current terms. I like smf's proposal where the core is BSD and the drivers are something more restrictive such as (L)GPL; it preserves the intent of the current licensing while eliminating the less-desirable features.
But ultimately nothing may happen, so keep playing MAME.