There is no unimportant dimension in sizing a bike .
Seat tube length is not all about seat height, on a level top tube frame it also determines the drop to the handlebars. A recreational, older or touring rider is most likely going to want the top of his handle bar within a couple of centimeters of his saddle height. That gives a rider a more upright relaxed position for cruising while still having the drops within a useful range for an aerodynamic position. Seat tube length (or effective seat tube length) is even more important now that nearly all new bicycles have threadless headsets which offer very little stem height adjustment. Top tube lengths can be adjusted by stem lengths that range from 60mm to 150mm. and by handle bar reach.