I will try to explain my point of view on X as opposed to 1♠ using a couple of your own quotes from other threads:
Mikeh said:
At imps, the cardinal rule, when choosing a final strain (as opposed to level), is safety. If game is out of reach, aim for the safer partial, not the higher scoring one.. and carry this theme through game and slam decisions.
So aren't we more likely to be in the right strain by starting with an immediate double? We actually win automatically in 2/3 chances (whenever partner bids hearts or spades), and it is entirely possible this hand actually belongs in diamonds.
mikeh said:
So I opt for the slight overbid of 2♠. While we may end up too high, we will always find the right strain, and, when it is close, it is usually better to be in the right strain one level too high than at the right level but in the wrong strain.
In both of these quotes, you appear to state (my loose interpretation) that we are always better off playing in the right strain. Imo, an immediate 1♠ bid really should say "I think this hand will play best in spades unless you (partner) have a better place to play" and yet, at this point in the auction, there is absolutely no reason to believe this to be true. Partner may well have a better place to play on this hand, but have no reason to bid over 1♠.
In fact, I believe that if you were to run another simulation, you are likely to find that 4♥ is the most likely game prospect, not 4♠ (and 5♦ isn't entirely out of the picture, either).
On this particular board, these are the original combined N/S hands
Change the North hand just a fraction to either:
or
Obviously 4♥ is a reasonable game on either of these hands, and yet....we may be passed out in 1♠ if we bid 1♠ instead of immediately doubling. Because of this, I much prefer putting both majors in play at my first opportunity to do so, rather than at a second chance later in the auction........that may never come. Of course, the possibility also exists that you may well be going down in 1♠, where even if you have no game, you may be making a partial in hearts or diamonds.
The only assured way of finding our likely best fit IMMEDIATELY is to double. While partner certainly may bid diamonds, this hand is still good enough to force for one more round by bidding 1S. But he may always bid 2D over the X, or 2H, or 2S, or 3D or 3H preemptively. It is these types of hands that the X is much more likely to cater to and improve our subsequent auctions (again, just my opinion), than the hands where partner happens to be a weak 3-3-3-4 and is forced to pick a 3 card suit.
If, by chance the bidding is at 3♣ by the time it gets back to me, with partner having failed to bid, I still dont have to bid 3S (as someone else blindly suggested), I can still X again for takeout, or decide that its the opponents hand and pass.
In fact, I have been so overwhelmingly convinced that X is the correct first call at IMP's, that in my current partnership, we will double on any 5-4-3-1 hand without any HCP requirement beyond being a normal takeout double. It took a long time for me to overcome my sentiments regarding the advantages/disadvantages of doing so, but the overall results have proven (at least to me) that this is the best way of treating these hands.
Note, I am only convinced of this @ imps, where the scoring differential between a minor suit contract and a major suit contract isn't significant. At MP, since majors outscore minors, and higher scoring contracts win higher percentages, I would still prefer to bid the major first unless the hand is actually strong enough to double then bid.

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