Hey Old Fart,

Friendly speaking, it's not academic. It's basic stability. It also doesn't matter if the thrust is in the front or in the back. Toe-in can drastically help a tail dragger:

Consider a model with toe-out. If and when the model turns a tad bit to the right, the right wheel is now pointing even more to the right while the left wheel is more in line with the centerline of the runway. The left wheel produces less drag while the right wheel produces more drag, causing the model to turn even further to the right, and so on and so on.

Now consider a model with toe in. If and when the model turns a tad bit to the right, the right wheel is now aligned more with the runway centerline and the left wheel is pointing too much to the right. The left wheel is now producing more drag than the right wheel, causing the model to turn back to the left. We are only talking about a few degrees here.

Toe-in produces stability (the concept that the model returns to it's initial position)
Toe-out provides instabiliy.
Parallel wheels provide neither, they are neutral. If and when the model turns a tad bit to the right, both wheels are pointing to the right and neither creates more drag, so the model will continue to go in that direction.

Fun stuff.

Kurt