i've played for like 10 years and i've only suspected a shaved bat maybe 5 times if that.
Man, you're either extremely naive or play in an extremely honest area.
As for the OP, I've played in quite a few different states and and it definitely does vary regionally, or from area to area.
More common in some areas than others so it does tent to be contagious..
Kinda like when you play a team that is notorious for it. I think about half the guys on teams like that never would have swung glass if they weren't playing with a bunch of dudes that do. It's the same way in different areas.
But overall, I'd say the actual number is around 10-15%. 1 or 2 per team on average.. Some teams where almost all swing glass, and some that don't have anyone that swings them.