Personally I think seat tube length is irrelevant. So long as you can stand over the top tube, saddle height is the most easily adjusted dimension on a bike.
More and more bike manufacturers are turning toward the stack and reach dimensions as the most reliable sizing guide. There is an interesting article about this approach
here.
Using these dimensions I have been able to compare all my bikes with interesting results, and now understand much better the dimensional differences between my roadie, MTB, drop bar tourer and flat bar tourer, and can relate these differences to my comfort level.
