Um, I did say I was SLF, Pugilistic Animus. I don't claim to know all the terminology. I'll stick to what I said here, though:
if you can fly high enough, at cruising (IAS) speeds you may be approaching localised supersonic airflow, which is rarely a good thing in a plane not designed for it.
I'm sure passing through Mcrit is no big deal in a plane built for supersonic flight, but I did say it was "rarely a good thing in a plane not designed for it".
I should undoubtedly have written more clearly, or maybe not have written at all. As I wrote earlier, Nubboy has also made the point I was trying to, and he has the advantage of knowing what he is talking about...
(SLF sneaks out of forum, with tail between legs)