i agree with this opinion. it would really be more meaningful if you can store the reward for use later. also i guess there would be a co-relation between the worth of the sacrifice and the worth of the reward. though a small sacrifice might produce a great reward, if the reverse is true, then high worth material would never be sacrificed at all, and it reduces your options and the attractiveness of the exercise.
Or consider this.. A while its possible to get same bonuses 5% or 30% bonuses for wall. No matter what type unit you use. Yet with better unit you get better duration. For instance EK could give you 60 days with 5% bonus. When spear only gives you 1 day. Combine this with possibility that 30% bonus dosent replace.. your 5% bonus.. but rather adds on it. Giving you 35% total.
If we use this method then both. small sacrifice and large one has a meaning. On top of that better unit gives you better change to get high end bonuses.
Sacrifice loot also seems very interesting feature, but maybe little hard to do. If something like this would be done with randomly generated barbarians. It could be much more interesting and fair.
It could work better that way. But I would hope they would become Lost barbarian towns then. Only thing is that It would be impossible to add tribe features to that. Not without benefiting multies.