Got to thinking general philosophy. I have been working a lot on the idea that I have had regarding two-way strong openings, with 2D handling strong hands with 4+ spades and 2C then reserved for hands with fewer than four spades. This structure seems to work wonders. So, why does this work so well?
Well, think about opening structures generally.
The strong opening bid is usually a problem for any system. Precision 1C sequences suffer when the opponents intervene (as they will quickly do). Standard 2C sequences suffer when the opponents occasionally intervene, but that opening itself preempts us.
The problem, in both instances, is strain. Even when strain can be resolved, and it often cannot, strain is often not resolved until we hit game level.
As a result, people tend to open a lot more hands at the one-level. Why?
The issue, again, is strain. People are less concerned about a pass-out, or about not showing strength easily, than not showing pattern and thereby losing strain evaluation.
The key to strain and shape is the start. An amorphous start, with little strain restriction, requires more "unwind" time. As such, these openings carry an instant message to the opponents that preemption will likely work because of the intense need to unwind.
In contrast, the more message about shape you can deliver in the first punch, the better suited you are to handle preemption and the less unwind you have remaining, as to shape and strain. The earlier strain can be resolved, the better level can be agreed.
So, what is the functional benefit of a spades-not spades split of strong openings? Strain resolution. This is sort of like one-level openings, crunched. Imagine playing that a 1D opening showed 4+ spades and that a 1C opening denied 4+ spades. That's not as effective as far as strain as 1S for 5+ spades, 1H for 5+ hearts, 1D for 4+ diamonds, and 1C for 2+ clubs, but it is a LOT better than 1C for anything. By dividing the world into two types, you have doubled your available tools to unwind the hand. However, and perhaps more importantly, you have also already said much about one suit, spades, the perhaps most critical of suits. Boss trumps.
So, the theory seems to be to refocus strong openings out of a pure strength-based opening structure to gain more strain-based opening structure. In all other opening bids, theory stresses strain primarily but strength secondarily. We do not have any serious systems that, for example, divide 1C as 10-13, 1D as 14-16, 1H as 17-18, etc., except as jokes. But, in all major systems, this strain idea is lost as to strong, forcing openings, whether 2C, 1C, or 1D. Of course, the ideal strain-based strong system is "Strong Two's," but be serious! No one does that any more! LOL
That said, it seems that a middle ground might well be the best approach, with two strong opening bids, giving up merely 2D as an alternative opening bid, to get back some of that strain focus that is so critical to every other aspect of bridge theory and bridge bidding.
I mean, if you are willing to play conventions like Bart and Support Doubles, which are largely motivated by strain focus, then surely you can see the benefit of re-introducing strain focus into strong sequences.
You find, as well, that strain focus actually increases the number of times you make strong forcing openings. Why? Without strain focus, you are handicapped. As a result, you avoid strength-first openings for strain-first openings, because strain-first makes more sense. However, if your strong openings are strain-first defined, then that primary concern is answered and you now have room, breathing space to unwind.