Each team has their own situation going on. I'm not opposed to a developmental QB with long-term upside if we like the prospect. However, if you break down those teams you've listed I don't think many of them have actually been helped in 2023 so far by carrying a higher quantity of QBs.
Cards: Without Murray it makes sense for the Cardinals to try a variety of QBs out. Note that they got rid of McCoy when they decided to go with Dobbs. Dobbs had 2 career starts and Tune is a very raw prospect, so in my view they were smart to carry a veteran QB on the PS just in case. If they get more confident in Dobbs and Tune then Driskel's job could disappear.
Rams: How did keeping Bennett help the Rams exactly? They certainly haven't rushed to replace him. At any rate, he was obviously there so they could kick the tires on a potential development QB in case Stafford is nearing the end, not because they needed another emergency QB for this season.
Bengals: The Bengals only signed a third QB after Burrow's injury so that works against your position. Moreover, when their PS QB was poached they were able to go out and sign another guy. And then they were able to sign a fourth QB when they weren't sure about #2 or #3. Despite all that, Burrow played hurt in week 3 since they had lost both games without him and otherwise the Bengals would likely have started 0-3.
Raiders: I do like the Raiders keeping a third QB, but that's because Aidan O'Connell is a developmental guy who I quite liked coming out. He's on the Raiders roster now because they want to see what he has long-term. I'd be all for adding him if he becomes available, but that's because I like the prospect.
Saints: The Saints are an example where it clearly didn't help, because their third QB was suspended for the first six games for a failed drug test. When Carr got banged up they were just able to go sign Jake Luton off the Panther's practice squad.
Jets: The Jets were able to promote their PS QB (Tim Boyle) to the active roster, but how has that helped them? He hasn't played despite Zach Wilson being the worst QB in the league through the first three games. This Jets team is not a good team without Aaron Rodgers and having two backups did not soften the blow of his injury in any way.
Colts and Panthers: These are examples of why having good backup QBs like Gardner Minshew and Andy Dalton can be helpful. The Colts keeping Sam Ehlinger around as a third QB hasn't helped their season any yet, and the Panthers don't even have a third QB right now because the Saints just poached their PS guy. This also highlights the downside that even if you do carry a good development QB on your practice squad, they can always be signed by somebody else.
49ers: Purdy is coming off a major injury so I agree it made sense for the 49ers to hang onto Brandon Allen. It hasn't helped them yet, but it's a good risk mitigation.
