While there are Olympic crutch races, this doesn't apply to most of us. We are going to go at a snail's pace… or perhaps a bit slower.
Then there's the chafing of your underarms and sides, not to mention the fact that you've lost the use of your hands altogether.
Up and down stairs is a complete nightmare. And speaking of uneven surfaces, those are just infinite accidents waiting to happen with potential for redamage (to your already-injured part) or additional damage (to as yet undamaged ones—"Yay!"… not).
While this may be so in some cases, it isn't for anyone with a knee injury since these knee scooters put a load on the very body part that needs to heal.
So, the facts that knee scooters are more stable, better on the arms, give you back the use of your hands, and are a speedier form of transportation mean absolutely nothing to the knee-injured patient who can't use them anyway.