Even if you could write perfect assembly, sticking it in your program makes it more difficult for optimizations to flow across the assembly you have written because it is opaque to the compiler.

Better yet, imagine that the compiler has performed value range propagation [1] to determine that the range checks, after inlining, were redundant or simply elidable. This would be infeasible if it were written in assembly.

[1] http://llvm.org/devmtg/2007-05/05-Lewycky-Predsimplify.pdf