I believe it already works that way for mouse buttons, but I don't think it currently applies to the number-keys.

Its kind of a minor change, maybe some builds rely on chain-casting short CD spells from 1-4 and will benefit from holding it down, but this seems unlikely to apply to a boxer. For instance, if you could say bind tempest rush to 1, and held down 1, and tempest rushed everywhere while saving left/right mouse for more situational abilities.

In either case it's not a huge change to the way we do things, since effectively you could achieve the same effect by just mashing the keys over and over, and we are likely to continue to do so for most builds because of periodic abilities that we don't want to cast immediately when they come off cooldown (e.g. once per fight abilities).