Jonathan Randall said:
No need to be disrespectful.
I believe he meant that animals don't intentionally exercise for strength and fitness in the same way that humans do and that since we have both bodyweight exercises AND weights to work with, why limit ourselves to one?
Animals also don't lead the sedentary lifestyle that we do, or eat the poorly constructed foods we typically do. When your body is trained to burn fat instead of sugar and you lead an active lifestyle, then every motion leads to greater physical fitness. You even burn fat as you sleep.
You can see the difference in pet dogs. If you feed your dog well and allow it excercise, it will have the energy level of a wild animal. It will move with grace, and no human could ever hope to run so fast. However, if you have a sedentary dog that is overfed and/or poorly fed, the animal will develop joint problems, overall poor physical health, and typically look terrible. I feel so bad for those dogs, because unlike all of us they have no real choices.
Those of us with desk jobs absolutely need to have an excercise practice, be it martial arts or weight training or yoga or Circular Strength Training. There is definitely schools of thought that say you should cross train lots of different excercise programs.
http://www.crossfit.com/ is a great source for this type of program.
Other schools of thought would have you follow a specific program to a mastery level before you try to branch out and confuse your body by working towards often opposed physical goals.
Each has their benefits, and you needto research and choose the path that's right for you. Much like choosing a martial style, and much like getting good at anything in life.
Whatever you do, it's important to enjoy your excercise practice.