It's the kind of disability that benefits the character. It emphasizes his mind and lets him remain in a supporting role, which makes him a better mentor character. He's able to be a powerful figure while having a good reason he doesn't solve issues himself.
I had always assumed that being crippled was a sort of power trade-off for Professor X. Super-powerful mind, not-so-cooperative body.
It's the kind of disability that benefits the character. It emphasizes his mind and lets him remain in a supporting role, which makes him a better mentor character. He's able to be a powerful figure while having a good reason he doesn't solve issues himself.