The key is that Mozilla is betting hard on Emscripten/asm.js building marketshare/mindshare into the future, and being perceived as being exactly as performant and reliable as C/C++ running in a native process. SIMD.js should of course be able to run from non-asm.js code... but that's more of a bonus (since that's an almost-strict subset of the work required to get it to work on asm.js code, AFAIK).
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The key is that Mozilla is betting hard on Emscripten/asm.js building marketshare/mindshare into the future, and being perceived as being exactly as performant and reliable as C/C++ running in a native process. SIMD.js should of course be able to run from non-asm.js code... but that's more of a bonus (since that's an almost-strict subset of the work required to get it to work on asm.js code, AFAIK).