There are a few problems with the baseball bat method.

First a bat is very inconsistent. The lip of the fender won't be a consistent angle, and if you are rolling your fenders to gain clearance, a consistent angle is what you need.

Secondly, it's hard to account for paint cracking and small dents created by spot welds on the fender lip, since you won't be able to watch the inside of the fender while it's being rolled. Not to mention it's hard to just "roll" a fender with a bat, without pulling it too. If you aren't running crazy wheels, there's no need to pull, it will not achieve the flush look, most are going for. There is also no way to control the amount of pull, you are at the mercy of the bat.

Also something to consider when trying to just roll you're fenders with a bat is that a bat will generally not get the inside lip rolled completely flat. This is necessary when you need to gain as much clearance as possible.