I worked on a 3d web based modeling app in my last job (Autodesk) that was based on asm.js. (and this was a legit solid modeler with heavy CPU usage - not a toy). It was basically 50% speed compared to the desktop version, which isn't ideal but totally viable. WASM is definitely welcome but you could do the same things in 2015 with asm.js. Honestly I think the biggest selling point is decreased download size. That was our big pain point with asm.js