Excellent article Steve. I had to think hard to come up with anything else on the topic to contribute. I know I will have to relent and get a Diamondglass some day.
I would also consider the general speed of the water when selecting both the weight and length of the rod. Were I fishing a calm spring creek with mostly flat water and lots of dry fly opportunities, I wouldn't hesitate to use my 7' 3wt. However, if it was a steep freestone tumbler like Spillway Creek of the LMF (Lower Mtn Fork river near Broken Bow OK.), and I was expecting to fish nymphs and shot, I would rather have my 9' 4wt. The extra oomph of the 4wt also helps steer hooked fish away from the faster water.
One point hinted at by Jason Borger jumped out at me too.
If the longer rod means I can crouch behind big rocks and still get decent line control, then I'll grab that rod.
I also like a longer rod for the fact that it gives me more reach and roll-castablilty when crouching to stalk fish. I have fished many creeks which have a nearly complete canopy of trees overhead where I find myself squatting down and roll casting the whole time.