I am reluctant to be a dorky engineer with this one , but perhaps it may shed some light.. The combined stresses in a fishing rod under load, either during the cast or when retrieving a fish are made up of a combination of both bending and torsional stresses. These two stresses combine to give you a total stress, which is something that an engineer takes into account when sizing shafts for their proper size for a particular application. (A loaded fishing rod is really no different than a shaft under bending and torsional loads.)
Now for a given fishing rod that you're using, the applied bending stress during the cast is a function of lure weight, how hard you cast and how long the rod is. When you hook up to a fish, the bending stress is a function of how hard a fish is pulling, the angle you hold the rod, and how your drag is set.
But, and this is "the big but", the amount of torsional stress on the rod is determined by the load and the application point of the load away from the centroid (centroid is the center of mass of the cross section of the rod at a given location). So by aligning the guides with the spine you can reduce the amount of torsional stress by keeping the load application point in a vertical plane close to the centroid. And this will reduce the level of your combined stresses in the fishing rod. For lightweight, thin wall thickness, casting rods that are prone to breakage, this is in my view, a critical concern of rod building.
That is my opinion of why I always put the guides on the spine.
Ed