There’s generally a tradeoff in control systems between stability and performance. Which is to say: by definition, the more stable a system is, the harder it is to get that system to move away from its setpoint. The classic response to a program manager who wants the system to be “as stable as possible” is that a boulder sitting on the ground is stable, but you probably don’t want it.

If you want an aircraft that’s incredibly responsive, then you probably want an aircraft that’s just this side of unstable. Or, in the X-29’s case, an aircraft that actually is unstable and being actively stabilized by the flight computer.