Every year I buy a used bat from Play-it-again a size heavier to use for practice and prepare for the following season. Now I've read a couple of articles saying heavier decreases bat speed and lighter was the way to go? I'm old school and have always used heavier to get stronger but was thinking about going lighter and wanted to hear some thoughts on this idea first.
I just bought a 33/30 for my 12 year old for the cage. Definitely the way to go. We don't use weights in the on-deck circle for the reason you mention, but I certainly don't agree it should apply to cage work.
I think the studies you're referring to are in reference to using a heavier or lighter bat as a warmup immediately prior to swinging your normal bat, so in the on deck circle during a game. Working with a heavier bat in BP, especially for a youth who will eventually have to move up to that weight in the near future anyway, seems unlikely to be a detriment, at least in the long run.
I believe the info you're referring to regarding decreasing bat speed is more toward using a weighted bat or warm up ring/sleeve prior to an at bat. Research has shown that doing this actually decreases bat speed immediately after.
Using a weighted bat for training to increase strength is a different scenario. I believe that this is a good strengthening exercise as long as the heavier bat doesn't introduce bad mechanics. I watch kids all too often using bats that are too heavy for them and the result is REALLY bad mechanics. If not corrected, it can create bad habits for years to come.
We've always gone back and forth with a quick stick and heavy stick during off time, and alternating a heavy bat into cage and tee work - once bucket with, one without etc..