nabla: Mathematically, it's probably correct for the trailer to double at the first opportunity in all post-Crawford situations. However, BG is more than mathematics and psychological factors lead many to try to game their opponents in situations where the opponent does not have a free drop.

Of course, the biggest psychological factor of all is the risk that you may forget to double altogether, winning 1 point rather than 2 and giving your opponent a free drop in a later game.

On your other point, I don't think there are any first roll/second roll combinations (someone will likely prove me wrong) that create such a gammonish position that the trailer should refrain from doubling on that ground alone. In fact, this is likely to be the situation where an immediate double is called for even by those who tend to hold the cube: if the opponent believes the position is gammonish, he may make a mistake and drop.