MIPS are a general indication of performance at a given clock speed. without understanding its limitations, it is fairly dangerous to use.
for example, not all instructions for pic/avr are 1-cycle instructions. plus, with more instructions than most pic, avrs can potentially get more done with fewer instructions.
then, when you compare 32-bit chips (most arm chips are around 1MIPS/Mhz) with 8-bit chips (PIC for example), it may take multiple instructions for an 8-bit chip to do the same as a 32-bit chip.
etc., etc.