My confusion stems from the use of two different types of relay/forcing bids in the same convention:
2. The XYZ 2♣ bid is obviously different; it's a "forced relay" bid that virtually requires opener to make yet another artificial bid. In other situations, this type of relay is usually used when responder COULD have a weak hand that wants to drop the contract in the forced bid, but sometimes includes stronger hands. Examples I'm familiar with are transfers after NT, Lebensohl over NT interference or a reverse, and the 2♠ response to 1N with a long weak minor.
Here's the confusion...the 2♣ bid technically fits the above description because it could be dropped if responder wants to drop dead in 2♦, but it seems to me that this would be rare. So why are we wasting space with a forced relay bid here, as opposed to having opener immediately say more about his hand? I can think of several possible explanations, but none are convincing to me as of yet:
2. Want to conceal opener's hand. I'm not convinced because the opponents already know a lot about openers hand, and because it's not at all clear opener is going to be declarer
3. Is there anything else I'm missing?
And it seems to me that there is a cost to this use of space...it could push the identificatino of a 5-3 ♥ fit to a higher (perhaps too high) level. So after 1♣-1♥-1♠-2♣!-2♦!-2♥ (showing 5 I think), if opener merely has a minimum with 3-card ♥ support you can bid 3♥ no problem, but if opener wants to be in game across from the invite I think he has to bid 4♥ and then there's no room for cue-bidding if his hand was very strong.
Maybe the simplest way to ask is this: After an XYZ 2C invitational bid, why shouldn't we get rid of the 2♦ relay and have opener immediately tell more about his hand at the 2-level with close to minimum values, or at the 3-level with GF values (across from an invite)? This would be similar to the NMF/Checkback responses. It would also force any drop-dead ♦ responses to bid 3♦ (so similar to the 3♣ bid after XYZ.)
This all suggests one broader closing question...since the type 1 relays above are typically used in similar situations (with invitational or GF hands on responders 2nd bid...these would include NMF or Checkback, 4SF, XYZ, and even the Bourke Relay per the recent thread), is there a way to combine all of these into one "super convention" with similar response principles across all situations that would perhaps be less of a memory strain? Would this be helpful or not?