They are not relevant to measures of information storage. A megahertz is as irrelevant in this debate as a centimeter, or kilopascal --- except to the extent that it bolsters the argument that mega means million in all kinds of units, which nobody is disputing.
No they're not. They're very relavent for anything dealing with networkig and communications.
I recently programmed an atmega chip for a homebrew project and the frequency of the SPI bus was very relavent.