I think back to the last time I bowled without a thumb slug or anything (I'll mainly concentrate on the thumb here, as I've had grips in my ball longer than I've had a thumb slug, and the last time I had no finger grips was when I was throwing a conventional grip ball when I was 14), and the one thing I tend to remember boils down to one word: Friction.
As you're creating more friction with being in contact with more of the shell of the ball than with a thumb grip/slug, that friction will generate heat on your thumb. Normally that isn't an issue, but a few games down the track (say you're bowling a 4-6 game block or longer), your thumb will start to swell. And because of that you may tend to get a little stuck in the thumb or worse: skin starts to blister. Enough of that, and it will tear.
One day of that happening to me had me go to a slug and not look back, because I was not going to use Nu Skin.
Just something to think about when it comes to going back to without, and that is to think about if any additional room in the thumb is needed for the swelling, and if tape may or may not be needed.
BL.
