Spoiled onions and Tor exit relays

Exactly, and as the exit relay can't tell who it's relaying data for (at least without controlling a significant portion of the Tor network), it won't work. It's too high value a technique to use unless you're confident of your target.

I suppose if the exit monitored the traffic looking for e.g. a specific username being entered in plaintext on a certain site it could MITM, but even then you'd struggle, as IIRC tor switches circuits every 10 minutes anyway.