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Betza seemed to believe the BN was significantly weaker than Q (see cost table in Buypoing Chess, http://www.chessvariants.org/d.betza/chessvar/buypoint.html ). So Knappen's remark is at least a plausible guess at Betza's original reasoning, even if that valuation turns out to be incorrect.



Muller, it would be interesting to test your theory by letting pawns promote to something at least a full pawn weaker than a minor piece--perhaps a Wazir, or a backwards-facing pawn.  This should mean that it is no longer worthwhile to sacrifice a minor piece to prevent a promotion.

If you are correct that the power of the promoted piece has little effect because the threat of promotion rarely coerces the sacrifice of more than a minor piece, then the difference between W and WD promotion should be greater than the difference between WD and Q promotion, even though a WD is closer in value to W than Q.

On the other hand, if the difference is not noticeable simply because promotion is very rare and so its average effect is not large enough to measure, then changing the promoted piece to W should also have negligible effect.