I like top tube length as a means of frame sizing. In my shops, when I had shops, and then when involved in designing and importing frames, I gravitated to top tube length as the base for sizing. I was always a center to center guy, but as top tube angles began to slope, all seat tube lengths became useless for the most part. And handling went out of the equation IMO. To steer this back on topic (and prevent a personal rant), as I began applying top tube length to size, even to traditional classic or KOM frames, I found this to be a consistent means of measuring proper frame size. Over nearly three decades, I can't tell you how many times I heard riders comment that Italian bikes were just too short in the top tube. I think this was because they were buying/being sold bikes based on seat tube length, and if these frames, Colnago for instance, were sized center to top (seat tube or seat lug), then they were getting a bike that was a cm or two, or three too small.
Ensuring ample standover room and then going for proper top tube length, for me, became pretty much failsafe in terms of putting a rider on a bike that fit nicely and handled well for them. My $.02 worth, and probably overpriced at that.
Greg Overton interjecting useless information from near Denver, Colorado