It seems to me it's extremely common to work with vectors that are much larger than the host CPU's SIMD width. In that case you could get much better performanc

It seems to me it's extremely common to work with vectors that are much larger than the host CPU's SIMD width. In that case you could get much better performance letting the CPU schedule a large batch of work rather than trying to micro-optimize for multiple possible targets. Software is typically compiled for x86-64, but not necessarily for each specific x86-64 processor available. You often just get a generic precompiled binary. That's not really optimized for the machine you're running the software on. Maybe you want to have some restrictions on the size of the vectors you're working with to make it more convenient for the hardware, like "should be a multiple of 4x64 bits", but I still think there could be big gains with variable width in most cases.