Box2D for Java/C# is very useful (see Farseer in C# land), and can be more efficient than going through lots of interop calls. You can also hit DirectX directly using Sharp/SlimDX, which again, is also quite useful (if you want to write a game for DirectX, you are pretty much stuck in C++ unless you use these bindings).
Not to mention all the fellows using Unity (or previously XNA) to develop games with. Writeoff #1 always seems to be "not quite fast enough" and each high performance option can help take a chunk out of that population.
I'm not hearing nearly as much about MonoGame as I am about Unity (although I am hearing some promising things.)
I don't mean to imply Unity is 1:1 API replacement for XNA - it isn't, MonoGame is, yes. But I'd argue Unity has replaced (and quite effectively at that) XNA/MonoGame as a strong game building target for the masses. And it's not even arguable that, for me, Unity has absolutely replaced XNA, and MonoGame at this point is background noise.
And don't get me wrong, I wish this weren't the case - because the more I've used Unity's APIs, the more I've come to hate them. I've an extreme dislike of the out of date and buggy compiler Unity uses for some of those platforms. But Unity's got a broader platform set than XNA and MonoGame combined, which unfortunately for me trumps the rest. I'd unscientifically wager it has far more libraries and middleware being written with it in mind.
Maybe that will change. I would welcome such change. There have already been changes, even. But I'm not holding my breath - even if it does, it's too little and far too late for my day job.
"I don't mean to imply Unity is 1:1 API replacement for XNA" - this is what I meant. You cannot build your own engine on top of Unity, but some people don't need to.
Box2D for Java/C# is very useful (see Farseer in C# land), and can be more efficient than going through lots of interop calls. You can also hit DirectX directly using Sharp/SlimDX, which again, is also quite useful (if you want to write a game for DirectX, you are pretty much stuck in C++ unless you use these bindings).