Rumour has it there were plans to have some of the Diaclone repaints made into Decepticon cars back in the 80's, hence MP Exhaust is a sort of homage to this idea.

However when it came round to releasing Road Rage in 2002 Takara probably decided to make her an Autobot because it made more sense to them at the time, or some other reason. This is all just theory really.