I'm curious about the principles guiding (droll, I know) the choice of guides for fly-rods. I'd appreciate any feedback on the following:
What is the function of stripper guides? Why are they usually double-footed and what dictates how many a rod should have?
How did running guides evolve? Why do they sit so close to the blank (closest with snakes, less so with single-foots)?

Stripping guides do not have to double footed. In fact, from a pure fishing standpoint, single foot guides are much stronger than any fly rod they are likely to be attached to. But double footed guides have been around longer and were used in that capacity first.

By their very design, snake guides allow the line to come into contact with the blank. But, they were a great improvement over the traditional ring and clasp guide. Single foot guides, particularly those designed with fly rods in mind, attempt to keep the line off the blank, but not so far away that the loaded line on the cast will torque the blank out of alignment (the higher the guide ring, the greater the torque effect of loaded line will be).

A rod needs enough guides to adequately distribute the stress from an introduced load. Too few and compressive forces, which occur as the line pulls between the guides, can cause the blank to fail under less load than it would otherwise. Too many and you add unnecessary weight which is detrimental to rod efficiency. As guides have gotten lighter and lighter, we now find we can use one or two more than previously and thus provide good stress distribution along the blank's length without undue sacrifice in rod performance.

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Fabulous, that's exactly the kind of information that I was looking for.
Thanks Tom.