I agree with Meanstreak. The idea that bodywieght exercises cannot be used to gain muscle is wrong.
The reason few people gain size with BW is that theyn train easy exercises, e.g. push ups, for long periods of time. This in not good for strength.
However, using weights is just using large resistance, there's nothing about the weights themselves that make you gain muscle, just the resistance they cause. So it is possible to gain weight using your own body as resistance. Try for instance, one-arm, one-leg push-ups. Or maybe one leg squats. Then try elevating your legs for one-arm push-ups. There are millions of ways to increase resistance for BW exercises.
Here's the downside though: With weights you have far more control over how much weight you lift- it is more scientific and more quantifiable. BW excersises, although being pretty good, do not have the same efficiency as weight lifting, since you cannot isolate muscle groups easily.
BUT... If you want functuional strength, then I think BW exercises are the only way to go. They get your brain used to utilizing all the muscles in your body for a desired action, unlike weights, which while building muscle, do not do that much for functional strength.
Thats exactly what Charles Atlas tried to say. He got prosecuted for fraud.
Training with weights is alot more practical than trying to do handstand one handed pushups.
Blue Belt in WTF