I'd say if the reserves are so tight that the flight can't continue then the captain has already neglected his duties. This instruction is, after all, not necessarily binding for the rest of the flight.
Not complying and causing an TCAS RA (or actually banging into another aircraft) would cause more hassle than having to do some in-flight replanning.
I believe that's what the route-reserve bit is about, non-optimum levels and different than forecast winds.
Oh, how that Air France comment sounds familiar!