I guess everyone more or less does either cross train or engage in other activities to either enhance their primary art or to fill some other physical/mental/spiritual void.
Reza K - I recently had a discussion with someone who noted that all karate kata begin (inherently) defensive. I personally couldn't think of a kata (Washin-ryu & Goju ryu)to contradict that statement. But I wouldn't say that the goal of karate is to protect the attacker.
For myself, I engage in cardio and resistance training 3 days a week (ongoing activities) but also karate & boxing sporatically (crosstraining.
Anyone doing Tai Chi? I did for about a year a long time ago and found it really helped my karate.