Pete, I think if you watch your trigger hand carefully, while dry firing the gun for example, you will see that you do move it--although very slightly. Some people believe this (minimal) movement is easier with a straight grip than with a SPG or PG. That being said, the vast majority of American doubles were made with one or the other of those.
Where I think a straight grip does help, at least some, is with people who have relatively short fingers and a problem banging their middle finger on the back of the trigger guard. A straight grip moves the rest of your hand farther away from the trigger guard, especially when compared to a fairly tight PG.
There's also the school of thought, preached by the late gunwriter Don Zutz, that a straight grip should be paired with a splinter forend, and a PG with a beavertail. He felt that kept the shooter's hands more "in line". But once more, most classic American doubles were made with a PG/splinter combination.
If you work at it hard enough, you can overthink some of this stuff.
