You see, this is why the idea of operational range (OR) works so well, IMHO

! Imagine this scenario in the game:
You've got your eye on a massive oil supply near an enemy city deep in its territory (sound familiar?

) Anyway, you send a 'Stack 'O Death' of 10 tanks and 8 infantry deep into enemy territory. OK, say that the tanks have an OR of 6, and the infantry have an OR of 8, but the city lies 10 hexes inside enemy territory. Part way along your invasion path, you build a fort to act as a 'supply point'-as well as a place to 'heal' your troops. You then continue towards your main goal. The target proves more difficult to capture than you had anticipated, so you dig in for a protracted siege. Whilst you're doing that, though, an enemy unit has either (a) captured your fort or (b) placed itself on top of your 'supply line' (think embargoes in Civ3 to get an idea of how this would work). Either way, your units are now out of supply. Each turn your tanks will suffer a degradation of 20% to their firepower, armour and morale (4 hexes outside OR*5), as their ammunition and fuel and spare parts run low! In addition, each turn they have a 20% chance of losing a hp (or multiple if they use a larger hp system!) due to breakdowns and malfunctions. The infantry are a little better off, only suffering a 10% reduction to firepower, armour and morale per turn (2 hexes outside OR*5), and a 10% chance, per turn, of losing a hp due to desertions and malnutrition. These losses may be increased if they terrain is very hostile and/or the unit in question has already suffered injury.
Anyway, needless to say, these losses could prove devestating should the enemy launch a major counter-offensive! The only real MM involved in such a system would be to build the fortresses you need as supply points AND to keep a unit capable of defending it from enemy attack! This is a lot less MM intensive, IMO, than having YET another unit to move about with your military units!
Yours,
Aussie_Lurker.