In my youth I used only grips, but as i got older I changed to a slug in the thumb as well. Mine is purely for a comfort factor, I don't know how many of my balls would come back with giant chips taken out of the thumb and it was uncomfortable and never the same. I have never had that problem with grips or slugs.
I also hardly ever change my grips. I know of guys who change them every 2 months or so, some at the start of the season. I have a ball that I still use when the shot breaks down and they have the same grips in them from 1998 when I got the ball.
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Let in the DOGS - I've got a HAMBONE for them!
To all the haters, get over it!