Muller, to force a weakness in the pawn chain, and to have this as a criterion for piece introduction, is a good idea while the piece is introduced at a cost, making it strategically a relevant event, and it prevents the player from keeping the piece in reserve, which would give him undue defensive capabilities. For instance, depending on the powers of the external piece, it would be nearly impossible to succeed in an attack on the enemy king, while the defender couild simply insert the defensive piece at a suitable square. One cannot allow the player to delay the introduction of the piece while that would destroy the natural strategical flow of the game. In Burmese Chess, for instance, all pieces must be introduced before play begins. Otherwise, it wouldn't work strategically. In order to understand such aspects one must probably have a deeper understanding of chess resulting from serious study. Seirawan Chess only allows introduction of an external piece when a piece leaves the start square. In this way introduction follows naturally from the flow  of the game, and the player cannot keep an Elephant in the 'pocket' and introduce it whenever it suits him. This would be awkward and it would be impossible to lay out a strategical plan because you wouldn't know when and where the enemy external piece would appear on the board. So that's what's behind the gating principle in Seirawan Chess and in my variants. The reason why Seirawan introduced this rule is because he understands the principles behind chess.
/Mats