Why Not?
I guess a better answer might be arrived at if we pose the questions this way: "What is the penalty in a supersonic design?"
and the natural follow on,
"What is the penalty if I don't have a supersonic capability?"
I was heavily involved with a super maneuverable F16 test project in the 1990's. Employing thrust vectoring we had great nose authority at zero knots. The Tactical community would ask, would you rather have an off boresight missile capability or super maneuverability? My answer was always BOTH.
Throwing out a tired one liner, "speed is life" does shed light on the problem. Excess power can be used to turn or accelerate.
In a perfect world with appropriate ROE and BVR missiles a missile truck could do air defense. Historically, engagements end up in a knife fight. The ability to go supersonic might just allow you to disengage when the time is right.
Speed complicates the adversaries problems. Blow an intercept on a supersonic target and you may not catch it.
If you can go supersonic and your adversary can't, you can operate in regions he can't.
Gerz