This has already came up and gone back down. TW does not want to add more menu clicking BS for the player to have to do in normal gameplay. They may add changes to wages that simulate some effect of more cost for more advanced troops, however t4-6 troops already cost too much for thier performance level, so this need to be addressed before increases could be justified.
IMO the problem is much more that there's not enough good things to do with money and not that the player gets too much. Many player do not get that much and have a hard time with this aspect of the game too.
It doesn't give a moral buff. What it is, if you have say only grain for you troops to eat you get a -moral penalty, it's either -1 or -2(don't remember), if you have spartan perk on quartermaster you don't get this -moral.
HOWEVER if you have more types of food you not only don't get the -moral but you +moral too (up to +9 with max variety), so like I said, I would just take frugal and remember to buy more food types.
@black_bulldog is saying that after they get leadership to 100+ the moral bonus from leadership is enough that they don't worry about food variety for + moral. It's worth pointing out that this should mean he also doesn't need the spartan perk either at this point.
I don't think he means that the spartan perk helped him get leadership to 100 though. If anything I think the + max moral from food diversity would be a greater help in raising leadership as having more moral give you leadership (very small amount) exp daily. However the bulk of leadership exp usually comes from becoming a vassal and forming armies for more leadership exp daily.