Ok, I am going to begin by saying that I don't buy the "MAME as documentation" line. It is a complete retcon for one thing, and the only thing even supporting that statement is the statement itself. Trying to get hardware emulated right doesn't support that statement, it simply means they are trying to get emulation correct. There are however hundreds of reasons why mame ISN'T just about documentation and ISN'T totally legit. Need I mention hardcoded support for bootleg games, which is impossible to legally develop the way Mame is developed. Ok, enough of that, I got into with haze or one of the other devs a few years back, and don't really want to argue it again, I am simply setting you up for the position I am arguing.
There are plenty of reasons why MAME should completely remove and forget about 3D games altogether.
Prior to 3D games you could always find the best 2D games in the arcade, and thus 2D arcade games were generally superior to 2D console games from the same era. That is why we want to play the 2D games.
However, by the time 3D arcade games became widespread they were already lagging behind other 3D gaming options. Sure there are a few exceptions, but they are few and far between.
Now lets examine what we are even looking at here.
The majority of the 3-D games use specialty controls that almost no one is going to have (often analog driving controls). Is it really worthwhile to spend a lot of time on emulating difficult to emulate games that few people can properly play.
Many of the rest of the 3-D games are sports titles. Sports titles are fine and good, but you know and I know that they don't age well. Non-gimmick sports titles have a 1-2 year shelf life and then they might as well vanish, as the newer title is simulating the same game, but probably doing it better. If you are emulating sports games on your computer you would almost certainly get a better gaming experience by going down to Best Buy and picking up whatever PC game is out there for the sport of choice, heck buy last years title from the $5 bin, it will still be a far better gaming experience than that 7 year old version of the same darn thing you are trying to emulate.
For that matter 3D driving games while having a long active life in the arcade also tend to have a terribly short shelf life in general, works the same way as the sports games, the new game so closely simulates the same thing that it is superior simply by default of having better graphics, leaving only novelty driving games behind, and some of those are novelty enough to have real traction (Crazy Taxi), while most others are simply superceded by another driving game with glowing powerups and better graphics.
Is an arcade game running on PC hardware even an arcade game? What makes it one? Is "documenting" x86 hardware even necessary, or relevant.
What is left after the smoke clears and you look past all the specialty control games and sports games? Not a lot, not a lot, not a lot at all.
Stuff about not emulating games still making money for manufacturers is also not relevant, the industry of producing arcade games curled up and died 2 years ago, in case you weren't looking and didn't notice. Really, go to a distributor and see what is still available, and of those products that are still available how many are actually new. Checking the upright category on Brady Distributing's website there are a total of 10 NEW upright games available today (Ultracades, Namco reunion and other multirelease machines not counted in that total). What IS still available averages two years old, and that number is heavily propped up by games that are simply annual software updates like Golden Tee and the like that constantly have a NEW version of the same exact game. Oh, by the way, Brady is selling brand new 27" Ultracade units for $2375 now. Honestly (despite all Ultracade issues) that is better than most people would do building their own. Dynamo doesn't even make arcade game cabinets anymore.
Mame is darn good at 2D though, which is basically done. Actually, given that little nugget of info I think it would be better if the mamedevs did simply cease development altogether, at least then I could accurately give advice on 2D emulation setups without having to worry about the fact that last months mame version changed the way everything worked again, even though 2D has basically been perfect since .55 and now just gets slower with every release.