Historically you could never maintain armies like you can in MTW, that is you couldn't keep them alive and happy for very long periods of time. After a short while they'd get discontented and want to go back to their homes and crops. Here they can be kept active as long as you have the cash. This was the point in sieges - defenders actually had a shot at surviving because the attackers would get bored and frustrated after a while and want to go home. This isn't simulated at all - you can dig in against Constantinople for 20 years if you got the inclination and you WILL outlast it (Constantinople being the most advanced fortress in MTW I know of!!)

Realistically only the personal 'retinue' troops of a Lord could be expected to stay on a 'professional' basis. Troops like Urban Militia (thats the very essense of militia isn't it?) or Peasants would not last more than a few months.

Sieges will always lose so why bother?

Any chance that this will be patched or that army 'discontent' will be taken into account?