Why not keep hands up? I don't know-I am an old-stylist who was taught to keep his hands up-and teach my students to do the same. I will say though that at the Kukkiwon, (during Poomsae, not when we trained with the Samsung and Korean Olympians), we were told to lower our hands since keeping them up raised the center of gravity (picture high block w/left hand in left front stance, right front kick, stepping forward to right middle section punch-fairly common sequence in Korean/Japanese martial arts) which adversely affected balance.
After that training session, I can see why the Olympic folks keep their hands down and, I never thought I would say this, it makes sense for them to keep their hands down or in a lower guard position.
Personally, I will continue to keep my hands up, but I did enjoy the sparring. It brought some things to my attention that I need to work on. I am looking forward to continuing to train with you guys.