This means there is no need of any vectored semantics to be kept all the way to the GPU, and there is no need of expensive recovers (to use your words). And there is NO code degradation whatsoever.
The vectored code is much harder to optimize by compilers. Many of the standard optimizations are not even possible there. Others are much limited and much less efficient.
For example the microsoft HLSL compiler produces some arbitrarily-designed by them intermediate code that is vectored and as a result it is essentially un-optimized, regardless of their HLSL compiler efforts.
This forces all the GPU vendors to convert the code to scalar and then run full optimizations on it over again, including generic non-machine-dependent ones. This can cause high delays because
those optimization passes are extremely complex and slow.
Now if the intermediate code was scalar instead, much of the optimizations could be done beforehand (e.g. by the game developer instead of runtime on the user's machine). Of course the machine-dependent ones still will need to be done afterwards, but often they are the smaller part.
Another thing is that since the binary code would be standard, the conversion from GLSL to it would not be a vendor-specific task. So the OpenGL standard committee could form a working group to develop this compiler front-end
(it could be open-source, etc.) and relieve the individual GPU vendors from this burden. The vendors will only need to take care of the smaller task of converting this already generic-optimized scalar binary code to their specific machine's scalar binary code.
Please remember that one of the main purposes of a binary shader code is to minimize runtime delays from compiling/optimizing.
Another often cited purpose is to obfuscate the shader logic to make it harder to steal. Scalar code obfuscates much better than vectored because there the high-level vector semantics are already discarded and this makes the code much harder to understand.
If now you still don't follow my reasoning i give up explaining anymore.