Different customers have different preferences for their hardwood floors. Some like dark, some light dark and some like the mid tones. And, the truth is it also depends on the style of the home and the furniture.
It's interesting though, that currently, both extremes - light and dark are the most fashion forward and most popular. I love these 2 blog posts from Debbie, The flooring Girl, where she explains the popularity of the 2 extremes.