In AP you do get the 2 reserve slots immediately so you can keep some spare units there, at least until the vendors run out of that unit type.
If you have paladins and inquisitors you have 2 stacks with resurrection to reduce your losses, and you can do even more with the phantom and resurrection spells.
Despite that it did require a fair amount of running back and forth to top off my stacks though, since playing as a mage means every unit is needed.
I know there are plenty of options to bring back troops, but there are still scenarios where it isnt easily preventable at which case I wouldnt mind paying a LOT of gold to top off my stack.
Call it a crutch, but really its more the perfectionist in me and the feeling that well crap this next battle will be hard, if only I didnt lose those few troops from the previous battle. Now do I run all the way back to XXX (this mod also allows you to forget where XXX is for every type of unit) and come back or not? Or do I try that fight all over again? (not a good option either)
As far as the reserve goes, 2 slots isnt enough for lots of reasons. I like the option to be able to buy a new unit on the fly without stopping off at a base, so that sucks up one slot right there. Then if you have any extra unit you want to keep around (which is really the main point of reserve imo) that sucks up the 2nd spot. Even if I gave myself one filler slot, my other 4 units are still at risk.
I could play the game without the Mod, its just not as pleasant after having the taste of the way it could be.