OK, I'll bite.
Let me start by saying that on many an occasion I wished I could do that very thing (but not allowing my opponent to do it at the same time ). Here, however, is why I feel that the current model works as is:
In terms of realism, I would say that the ability to do this would be a feat of logistics far surpassing Hannibal's crossing the Alps. This would amount to launching a corps or better sized attack, withdrawing them while engaged head-on with a hostile enemy, moving another corps or better up to the front and then launching a second attack.
In terms of game mechanics, can you imagine the blood bath that the Eastern Front would be with both sides having this ability? It can already be a bit of one as is. If both sides had this capability, I think losses would far exceed either sides ability to replace/reinforce them, so that by 1942 there would only number some 7-10 corps per side (Germany & USSR) hammering it out to decide the fate of Europe.
Just my $0.02 (so take it for what it is worth)
cheers,
Ray