I don't know that it would be "unbalancing". However, I could see it being used as an excuse to further centralize power and gold. (Gold is, in any case, power...) "You don't need a big estate, just take that 2% estate, and all your expenses will be taken care of by the war chest." Keep all the knights poor, and all the gold in the hands of the rich ruling class. (Although, I guess that could be considered an abuse.) I think that for this reason, allowing the war chest to cover 100% of all expenses would be a mistake.