It helps to look at every idea from the point of a company which is out to make money and put your idea into a broad category:
- Makes the game more interesting. (They care. Players who are interested will spend more money.)
- Makes the game easier. (They don't care. they don't want the game to be easier because if it's easier players will spend less money
A badge bank would be a slight positive motivator for players to continue playing, but means that only one player really needs to be involved to complete things while the others just make as many badges as they can as fast as they can. That one player won't accidentally put the badges in the wrong way point and waste them, which might cause the person who originally made the badge and mistake to spend diamonds to correct their error. Most of the players won't be constantly reminded of the FA's score and tempted to pay for more badges to improve it.
There is practically zero economic incentive (in fact, a negative incentive) to invest programmer time in a badge bank.