In my mind, a crutch plane can take one of two forms; easy to fly, hard to screw up in (Spit 16), and very high performance, but harder to fly, requiring a more experienced pilot (Yak-3, K4, Spit 14, etc).
Now the latter can sometimes be a crutch, sometimes not. For me, I lack the gunnery skills to fully exploit the K4, so I wouldn't call it my crutch, not yet. The Spit 14 would be my personal crutch.