Should knee caps be worn as a preventive measure in Tennis?

There is no magic bullet, I am afraid. I don't think you mean actual caps, like some doofus here suggested, you mean knee support/braces. Unless you have a bad ACL, you problem will be the wear of cartilage and patella tendinitis. Essentially, you will suffer from inflammation. To that end, icing your knees after the game will go you good. You can try anti-inflammatories, if you feel the pain. For some people, starting a daily dose of chondroitin/glucosamine supplements works very, very well. You can also do some specific exercises to strengthen the muscles around the knee, which will help stabilize it. But, ultimately, all will fail - especially with your daily schedule. I would strongly suggest switching to clay courts, perhaps playing more doubles, and resting some days. It would delay the inevitable.

Knee caps can't prevent joint damage. Knee caps are only used to prevent damage to the skin, like...roller blading. Long term damage to the joints can only be prevented by wearing shock absorber shoes. Just like how getting better strings will help prevent damage to your elbow, but wearing elbow pads can only prevent scrapes or things like that..

"COULD prove extremely harmful...in the long run" Any leg pounding on a hard surface for long periods could cause damage. If there is nothing wrong with your knees now I wouldn't worry about it. I teach and play 4 hours a day, 6 days a week and have no knee problems even over 50 years of age.