The more I see problem after problem crop up, the more I like my ideas for using 2♦ as a second strong opening.
An example hand, from BBF:
Opener: ♠KQJx ♥AJxx ♦AKQ10x ♣--
Responder: ♠Axxx ♥Qx ♦xxxx ♣Kxx
The discussion was as to what should happen after an uncontested start of 1♦-P-1♠. One stand out option was a 4♣ preempt, which excites Responder little. Even if Responder does have interest, his only option seems to be a 4♥ last train call, showing general but insufficient interest without a diamond card. Maybe that works.
Fred G. suggested using a rebid of 4♣ not as a Walsh Fragment (roughly, a strong 6-4 game raise), but rather as a general, all-purpose super-power slam invite. I have no idea what that means, or what Responder should do about that.
2♦ (Strong, artificial, forcing, with 4+ spades)
2NT (GF, with spade fit. Sets trumps. Promises better than a "minimum game raise.")
3♥ (Shows hearts and spades for cuebidding purposes. In other words, Opener wants to hear about spade and heart honors, whether Ace, King, or Queen)
3♠ (One of the top three spades; obviously the Ace)
3NT (Serious -- tactical, to get under 4♣)
4♣ (club control)
4♦ (club control not enough -- I have a diamond control)
4♥ (heart Ace, King, or Queen; obviously King or Queen)
At this point, Responder has gotten off his chest the club card. He has also been able to establish trumps low enough to have cuebidding occur. He has been able to show the heart Queen. This is good. Opener can now make a more informed decision.
An alternative choice early on leads to another decent sequence.
3♣ (general non-informative cue -- asks for cuebidding of controls to start)
3♠ (no first or second-round red control, one of the top three spades -- the Ace obviously)
3NT (serious, with sufficient red controls)
4♣ (club control)
4♦ (diamonds really controlled)
4♥ (tertiary heart control)
Because Responder cannot have just the spade Ace and club King (would bid 3♥ over 2♦ as a minimum game raise in spades with at most one internal card and one external King), Opener knows that Responder cannot simply have a doubleton heart. There must be some other redeeming value.
Notice how Opener has no problem with this three-suited hand. In fact, a GF is set up with general strength parameters and fit established and the entire three-level still available for slam probes and cues.
Had Responder bid 2♥ over 2♦ (waiting, no spade fit), Opener could rebid 3NT to show 4441/4450 and not super-huge (would bid 4♣). Responder could then agree diamonds or hearts below game in either suit, if he wanted.