The last issue is the inference from the redundancy of a Jacoby 2NT auction alternative. This part has been the most difficult to assess, for several reasons. First, I have a gut dislike for Jacoby 2NT, even though I play it. I have a tendency to avoid Jacoby 2NT, whereas it seems like many have a tendency to jump right to Jacoby 2NT. I'm not convinced that Jacoby 2NT solves the world's problems.
The second problem in describing the redundancy from Jacoby 2NT is in describing when you would use Jacoby 2NT, which in turn turns on the systemic responses to Jacoby 2NT.
From a theory perspective, it should be relatively easy to understand the issue with Jacoby 2NT. The normal responses force a two-way bash, to a degree. In the old style, there were two "bash" bids induced. First, one could bid a stiff or a void, jumping with the void. This is a "shape bash" bid. Second, one could use a quantitative bash of 3M, 3NT, or 4M, all showing HCP ranges without shortness. The newer school has combined the stiff and void bids into the non-jump, with a trick-source jump, akin to a Picture Jump. That's getting closer to my tastes.
The assessment of the impact of Jacoby 2NT will require several steps. First, I will describe the response structure that I currently use after a Jacoby 2NT raise. This response structure is not critical, as many similar structures do exist and are useful. However, it may be of interest to some, who perhaps are playing a much less involved structure and, accordingly, are finding that Jacoby 2NT auctions leave a lot to be desired.
The second step will then be to describe what hand patterns for Responder are best handled with a Jacoby 2NT response, considering the response structure.
The third step will then be to assess what impact this tool has on alternative auctions. For, it seems clear that a cuebidding sequence will never be selected when Jacoby 2NT is ideal. Thus, when a cuebidding sequence does occur, and when Responder's hand starts to look somewhat like a hand suitable for a Jacoby 2NT auction, we will then have an idea of what "Jacoby 2NT flaws" might have justified this alternative route.
As an aside, I had some difficulty deciding which came first, the chicken or the egg. One could decide to look at this problem in the same way that I analyzed "shape bash" splinters and delayed splinters. In other words, just as shape bash splinters handle problem cuebidding hands, Jacoby 2NT could also handle problem cuebidding hands. However, for some reason that I cannot quite articulate it seems that the reverse should occur when analyzing Jacoby 2NT. This may be thinking based upon years of being in with the "straight to Jacoby 2NT" school. However, it seems to me that Jacoby 2NT caters to bash bidding by Opener, and that therefore the reverse is true. One partner bashes, the other caters. So, because the structure for Opener is "bash," Responder should generally only bid Jacoby 2NT with a hand that is appropriate for decision-making after the bash.
It also seems apparent that Jacoby 2NT is a captaincy-grabber. I generally dislike bids that grab captaincy as your first call. There ought to be a very strong reason to make that move.