Mediums: best compromise or worst?

deriving from the j-10 and rafale discussion..
a number of air forces are moving to single fixed wing combat aircraft that will do everything.
Germany, France, USN?, etc
often these aircraft are considered medium aircraft as opposed to light (gripen, tejas, jf-17), or heavy (Flanker, F-15E, F-22, Pak-fa, etc).

are Mediums the best compromise?
or perhaps the worst, which might be way some are avoiding them and sticking to either lights, heavies, or a hi-lo mix.

it might be hard to categorize mediums,
but perhaps something around 9,000kg -13,000kg
and 15-18 meters in length

with that definition, some lights might cross into medium territory, especially newer versions F-16, and some heavies into medium, like F-15A