According to a Sport Science episode, a weighted bat or bat ring will upset a hitter's timing and result in slightly slower bat speed - even though it feels like a more powerful swing.
It's hard to argue against science.
So the takeaway from this is - use your normal bat (without weights) to warm-up.
Most players believe that swinging a heavier bat enables them to swing the regular bat faster because it feels lighter.
Yes, the bat does feel lighter, but is it really traveling faster?
The reason for this is that the strength you gain from using a heavier bat is not proportional to the muscles and joints that are involved in the swing with a regular bat. In other words, you over develop certain muscles and under develop others. In essence, you do not develop strength as it is needed in the usual hitting pattern.
