One is that from my view any swordsman in practice who cannot control his sword to the point if stopping it at the strike point is not very good. (whether 5mph or 90mph) Therefore with that amount of discipline and skill there is never the need for a tsuba. This ability to control the bokken, or sword for that matter, should be one of the primary aims before anyone tries 'fast high level sparring'
Secondly I say that the primary purpose of a tsuba is Not the protection of the hands. That's not how it came in to being and not therefore what it was for. They had gauntlets for that.
A few points.
First, gauntlets were a battlefield accoutrement. You wouldn't wear them on the street, or to a social gathering. The samurai class wore their swords *everywhere."
Second, without a tsuba, the hands become an immediate and obvious target. Sliding down the opponent's blade disarms him or takes his fingers off, rather than trapping you against his guard. That alone makes a significant difference in strategy.
And finally, no matter how good your control is, accidents happen. They happen more often as you approach your personal limits, but if you never test those limits, it's difficult to expand them.