There has always been the misconception in baseball and softball that if you swing a heavy bat before going up to the plate you will be swinging your regular bat faster after you take it off. In theory it sounds great, after all the bat does feel lighter. However, when you swing a heavy bat, you are training your body to swing at that weight. Obviously you are going to be swinging the heavy bat slower than you would your normal bat. So, when you remove the heavy weight and swing your bat at its regular weight you have now trained your body to swing slower. Your body doesn't know the difference between swinging the heavy bat or your normal bat. As far as your body is concerned it still thinks that you have the heavy bat in your hands. You will now actually be swinging your game bat slower than if you have never used the weight in the first place.
Adding a heavy bat weight will change the body's biomechanics. When adding any weight over roughly 20% of the bat's original weight your body's swing mechanics will be altered as well. WIth the bat weight on, you are now practicing a swing that is different from your "normal" swing. Instead, you should be reinforcing the good mechanics you have been working so hard to achieve. Don't let yourself or your players go up to the plate with altered mechanics and slower bat speed by using heavy weights on deck!
My suggestion instead, is simple. Use the weight to stretch out and get loose with, but do not take full swings with it. Take the weight off before you take any actual swings while on deck. Let your hours of preperation with mechanics and bat speed training take over in the game. And don't hinder it with a bat weight in the on deck circle!
Check out the video below for more on the subject!