I am presuming that the kernel in nearly all distributions is also compiled for vanilla 'Pentium' rather than using any exotic instruction that risk compatibility. Many Linux installations are on very old hardware that doesn't have any other uses. I'd like to actually use the fancy capabilities of my hardware. The MMX instructions are now something close to 15 years old. Even SSE is about a decade old now. (And then there is SSE2 and SSE3, but I don't have any processors that handle those...
) Also, there might be some services included in the kernel by default that I don't need and don't want for security reasons. It's been a few years but I did tinker with Linux a bit back in the 1.x kernel days. Then, you had to recompile to change quite a few options. Maybe that has been fixed?
