I think the monetary value of the board isn't really relevant to a games historical or collectable value. I have seen SF2 boards go for 15$ and there is no disputing its status. Additionally, the availability of a 99 cent PSX version of Soul Edge isn't any different from practically arcade perfect releases of any other classic game. There are differences that the competitive gamer would notice.
I will agree that Soul Edge is not exactly going to be a common pick for fighting game players. But it is not "Also ran", more like sleeper hit. It is highly respected by fighting gamers. To the right person, someone who may have played it in the arcades as a child or teenager, it may be very valuable.
The point that I trying to get across is this: Any cabinet, no matter what game it is, no matter what era, should get a measure of respect. For someone out there, it was "that game" that they played as a child. What seem to be the games that just came out yesterday may be sought after in the future. We shouldn't want to turn Mortal Kombat's into MAME cabs to play Defender any more than we would want to turn a Defender into a MAME cab to play Mortal Kombat.
Serious collecting of 90's era arcade games may not be as big as the 80's era, but it is still significant enough for people my age, 23, to want to make sure that the games I played as a teenager and child aren't completely forgotten.
To the OP: Don't let me harass you too much though. Go ahead and MAME it. But please, try not to do anything that couldn't be reversed.