It's a gimmick. It doesn't make the game more immersive (You need true 3D for that, not an illusion) and it requires a more expensive TV/Monitor.
Shaun Kennedy said:
3D isn't that niche, the success of 3D movie sales in theaters is pretty good, but the entry price point for at-home 3D is a bit high (though not inaccessible). I think the biggest issue though? It's not cross-platform friendly.
Basically if you're sitting the appropriate distance from your 3D TV with your glasses on playing your game on a console then it looks fine. Put that same game on a PC and you're going from a distance of like 20-30 feet to 2-3 feet away from the monitor, which is bound to have an impact on how well the 3D works. Very few people have 3D capable PCs at the moment I would wager, they are very much a novelty right now and don't have the presentation quality you would get from a 3D home theater which has a similar price point.
Actually, most people these days who have an Nvidia/AMD graphics card, have 3D-capable PCs. The problem in the monitor, as you need a 120 Hz monitor for that. Those glasses aren't that cheap as well (are they still required though?).