i thought i had read that you could buy the 3/4 adapter and use it in the 500 sharpener, but maybe i'm wrong - the drill doctor site shows the 750 being able to do a slightly different range of angles, so maybe the base unit is different. for bits over 1/2" i'm generally comfortable hand sharpening them, but for smaller ones i'm not. it's not just a matter of being able to grind a sharp point that will cut, it's making it so that both cutting edges are identical.
in the last couple years i've done more "precision" work, and i want my smaller bits to be as true as possible so the holes go as straight as possible. i've made a lot of smaller parts where i need to have multiple holes line up precisely. there's no oversize drilling allowed for stuff like this, and i don't want a bit where one side leads the other and the hole wanders or ends up slightly off. this is mostly for small stuff. if i needed precise holes that are larger i'd probably bore them instead of drill them.