As far as I can tell based on the literature and some common sense smell testing, the reasons for the Merkava’s back door are:
1. It permits the crew to board the tank without having exposing themselves to small arms and artillery fire by climbing on the tank to enter through the turret.
2. Easier to replenish ammunition than handing up to the turret and through the hatch. Also safer when under fire.
3. By discarding or expending part of the main gun ammunition load enough space can be freed up to permit the crew of another knocked out tank to be rescued and shelter on board while the tank withdraws to a safe area to unload them. There is some question as to how restricted the Merkava’s combat capabilities are this ‘pregnant’ condition.
The mortar is another piece that may have an interesting rational. Some sources claim that the principle use is for clear upper story rooms during combat in built up areas where the main gun cannot elevate enough to be used. I would bet that they have, or are developing, a round for clearing rooftops too.
Good post, these are some of the lessons learned from past conflicts, people tend to focus on the rear hull use for ground pounders due to lower level skirmishes that have been fought pretty much during Merkava introduction; Given a pure defensive posture this tank by far is the one I would rather be in versus some of the other models currently serving.