f/6.3 might be a tough sell at 200mm. Yes, it is only a 1/3 of a stop. But there's are psychological barriers at f/2.8, f/4 and f/5.6. Lenses spec'd with "whole" f-stops have a greater perceived market value even if 99% of people couldn't actually tell the difference comparing images at f/5.6 and 6.3. It is no accident that Tamron designed the 28-200 so they could print "f/2.8-5.6" on the barrel. Those f-stops are perceived as "better" by most folks when compared to a lens offering fractionally slower "f/3.5-6.3". Sales is all about perception.
