The B17 is designed to be ridden at a height equal to or lower than the handlebars.
Yes there will be people who ride them outside of these parameters but they are exceptions to the rule.

Sheldon says: "The B17 is most appropriate for cyclists who set their handlebars about the same height as their saddles, or a bit lower." (http://www.sheldonbrown.com/harris/saddles/brooks-b17.html)