I was reading this, and found an interesting little quote about womens knees.
Also, the knee is a visual anchor, says Barbara Pachter, an etiquette expert. People's attention will be drawn downward when they approach you, instead of toward your face where it should be.
It's an interesting way of putting it: 'visual anchors.' Now, she is not talking about frum guys then NEVER see knees, but regular guys in every day life. Even for them, it is something that grabs their attention. From personal experience, I think she has a very valid point ;)
The interesting question is, does the same apply for guys. Do women get distracted if they were all of a sudden to see a male knee.