Not a silly question. Not sure about it though - do you just mean a urethane or vinyl thumb insert (the insert is normally referred to as a "thumb slug"), or the interchangeable systems that allow you to use the same insert in various ball (Switch Grip and the like)?
The selling point of the standard thumb insert is a more consistent feeling for the thumb. The textures in a standard thumb hole can vary from ball to ball, and with a urethane or vinyl insert (the latter is softer and slicker, and less frequent) you get IMHO a much more consistent feeling between balls, and the holes are easier to tune.
Personally, I did not use any thumb insert in the beginning, but whan I had to adjust my thumb pitch after some time I tried a urethane slug in one ball and really liked the "better" feeling, changing the whole arsenal. I also tried vinyl once, but it was too slick for my slender fingers, and very dangerous with sweaty fingers.
The interchangeable slug goes one step further and allows you to use the same insert on any ball that is fitted with an insert adapter. Costs a bit more, some players love it, but it is a mechanical thing that might break or come loose over time - the latter is the reason why I stay with my standard slugs.
Hope this helps a bit?
--------------------
DizzyFugu - Reporting from Germany
Confused by bowling?
Check out BR.com's vault of wisdom: the unofficial FAQ section
Secrets revealed: What's a fugu?
