There would be many parts of the flight envelope where one engine isn't enough.
This is a question I've been wondering about for a while, and was also alluded to in a post about the Jaguar a few days ago - does the average multi-engine FJ have to meet minimum performance requirements at take-off along the same lines of thinking as civvy jets? Eg is the MTOW of (say) a Typhoon or Tornado limited by a requirement to meet a specified ROC if an engine pops at Vr?
My reading of the post about the Jaguar suggested that the Jag could not climb if it lost an engine?