the real full answer is:
if any one player on a team could contribute his full potential to his/her team, then the relatively low quality of the game logic will surface significantly; the decision is to cap the potentials of skills here-and-there, which may yield a somewhat more balanced game due to the logical (pseudo)randomness introduced with this decision. another reason leading to the decision is that randomized credit/evolution counts force players to chose from not-so-desired equipment/classes, yielding games with more variably distributed use of equipment/classes.
*sigh*
The real full answer is that tremulous is designed with an RPG element, such as in COD4, where a person who makes lots of kills gets perks, only in tremulous that is *much* more pronounced (and changed) to the point of being an integral part of the game. However, the accumulation of power that a skilled player can achieve can get so extreme, that there is little that can be done to bring down a determined killwhore, returning him temporarily to a less dangerous. Therefore, built into the design, is a limit on how many kills you can save at one time.
This is further compounded by the fact that wealth tends to be expended to attack the opposing base (an unfavorable situation), and accumulated when slaughtering the enemy in favorable situations.
Overflow credits, or any other share-like system are even worse. Instead of merely giving one player an infinite abillity to become a tyrant, it allows, through the killwhoring of one player, who may or may not be skilled, to level up the entire team, and let the team accumulate power, and then expend it all simultaniously on the opposing base. The *problem* with share is that it allows you to give your evos to your buddy so you can both rush.
As a side note: yes, it means that there will be a more even distribution of people who are "fed" and "poor" in public games, but that is not ultimately what this is about, IMO. The distribution (or rather, the expenditure) of wealth can be controlled better in a clan(m) match.
I by no means say the game logic is perfect, but I think you should make your own thread about "flawed" and "poor" game logic.