It would be extremely helpful for building and destroying blocks while midair. The latter is incredibly difficult right now, because you have to press the jump/ascend button repeatedly at exactly the right time and intervals, and it really sucks for destroying many blocks in a row.
This could be implemented either as a toggle that keeps you at your current elevation, or as a button that must be held as long as you want to stay at that elevation. Either would work, and both could be overridden by pressing the jump/ascend or descend buttons. A toggle would have the added benefit of preventing those times when your flying mount just randomly decides to stop responding and drops you to your death, as well.