Under ASA rules, at least, I believe there are rules that significantly restrict what can be used in the on-deck circle. The old "donut" rings, for example, are not allowed.
I also believe there is some evidence that, in fact, use of weighted items in the on-deck circle is actually counterproductive - useful to some extent as a training device, but not in the on-deck circle. Perhaps someone with more information on this can weigh in here. That seems to be counterintuitive, given what we see everyday in MLB on-deck circles.
Flash101 you hit it on the head. High School ASA they cannot be used. Anything that changes your hand path is not a good warm up tool. Why would you want to swing something that makes you drop your hands and change your timing when you go to toe touch or heel plant.
Using a warmup bat on the on deck circle does several things. If used correctly, the warmup bat simply loosens the muscles before an at bat. Never do you see MLB players swing a weighted bat at full speed. That would be counterproductive. Instead, they take the warmup bat and rotate the shoulders and take a few soft swings (they don't change their swing), if the bat changes the level of the swing then the bat would be too heavy.
There are studies out there that say warmup bats used before an at bat slow down your swing. I agree with this to some extent. If used properly it would not slow down your bat speed. However, if I decide to take a weighted bat and swing it at full speed right before an at bat, it most likely would slow your swing down do to muscle fatigue. I'm also making an educated guess that if you swing a lighter bat for 30 seconds at full speed it too will slow down your bat speed also do to muscle fatigue right before an at bat.
Bottom line is that there is nothing wrong with warmup bats if used properly. This is why they are very popular at the MLB level (ie Meiga Warmup bat, Dirx, bat sleeve, etc.). I've personally used warmup bats as a player and offer them to my players on my teams that I coach. It is the responsibility of the coach to instruct players the proper technique of using warmup bats (If you choose to use them). I personally like using the warmup bats during drill work during my practices and have my players personally train with them during the offseason. This, along with other drill work, will help increase overall bat speed by developing muscle strength and memory in the the hands, wrists, and arms.