Even when it's out and mature, there is some complexity around gracefully degrading when it is not available - it is going to increase frontend complexity dramatically I suspect, at least until browser support dictates that graceful fallback isn't necessary for the feature.
Most current WebAssembly compilation paths produce asm.js as an intermediate artifact. In the near term, backwards compatibility should be relatively simple: save both and load the asm.js code as a fallback.
