Well I can give you my daughter's experience that her biggest years of improvements in goal came playing on teams that were "lesser" ranked or lower tier for exactly the reasons you mentioned. The first year she was bumped up to the top team, she actually had one of her worst years of development because she would only see a couple of shots a game and the coach did not play the ball out of the back (many of the top winning A teams play kickball because they have the best athletes anyway). There just wasn't much for her to do, and when the shots finally came her way, she was far less confident and definitely not in rhythm. My theory is that around 16 it might become more important to be a keeper on a great team, but while the kid is still developing physically, being a great keeper on a decent team is a pretty good deal and will also help build confidence because the team will really appreciate him. Side note: it is ok to have your keeper play on a team that has a weak defense, but we found it was awful to play on a team that couldn't score. It's more ideal for keeper development to play with a team that can make up for any mistakes he makes by scoring once in a while. If the team can't score, every goal he lets in will feel like death. Just my .02.
This is my rationalization and experience with my 03 GK's situation (13 yr old, 2 years at GK, 5'8"). He loves his coach who is a GK coach as well. Likes the GK training and his teammates, but predominately plays on the B team both in the field (3rd best field player) and as GK, but get's moved up to the A and 02 B team from time to time to play GK (SCDSL). The boy takes something like 20 shots on goal per game and typically will give up 2-3 goals (most point-blank ... coming from in the box because his defenders are Rec level players that can't clear if their lives depended on it). On the one hand, he is has progressed significantly from last year (steals ground, gets above the bar, punches well on corners, etc), on the other hand he as zero trust in his defensive line so adjusts his tactics and play style knowing he has no support (not ideal). The unfortunate problem for this team is there is no speed and they typically get 1 to 2 shots on goal that have a chance (not counting the 3 to 4 from 30 yards out). Its only when they have the 04' Academy players "club passed" up do they score.
It can be demoralizing for a young GK to be on a bad team. My kid, however, seems to brush it off and doesn't internalize giving up goals, which is good. But, I'm left with the constant feeling that since his teammates are low level players, does he not push himself given the futility of the team's performance?
Sooo ... I agree. GK's can develop better on low level teams, but it comes with a few negatives depending on the psyche of the kid.