How does PNaCl compare in terms of performance to "native" code? It still has the compilation overhead, it still has a lot of the bounds checking… It's not clear to me that PNaCl will actually be much quicker than asm.js.
I believe the ideal (for users) would be to target NaCL natively, with a fallback to server-side PNaCL compilation, and an absolute fallback to PNaCL compilation/execution.