Jumpers/hunters etc. jam the heel down as a balance mechanism, however this renders the lower leg near ineffective for giving sensitive, 'invisible' aids, as well as forcing the lower leg forward. In dressage, the whole aim, in fact it is an FEI rule, is to have the aids appear invisible, and the whole picture to be 'effortless'.
Yes the heel MUST be the lowest point of the ride, it is down, but just enough to allow the flow of weight to travel down the back of the leg and to balance in the ball of the foot and heel, allowing for more of the weight to be in the seat than the leg.
As a dressage rider, ideally I want 99% of my aids to come soley from my seat alone, thus I need my centre of balance to be over my seat bones, not jammed down the back of my heel. I need to be able to apply 'invisible', sensitive aids with the leg, again this is not possible if the heel is jammed down, causing tension in the lower and upper leg.
Ponyboy, dressage stirrups actually aren't as long as people seem to believe. The ideal length is for the base of the stirrup to knock against the ankle joint when the leg is hanging freely. The stirrup provide a steady base from which the ride can apply subtle aids, work without stirrups is good, however it can also create tension in the leg and hip joints, and makes life much harder when you must apply a spur without it being seen by the judge!
Refer to this article for more detail on stirrup length