Ive not used thumb inserts for two years now and it's been great. My thumb systems is a 15/16" thumb hole with 2 pieces of tape at 2 o'clock  and then 3-4 pieces of tape at 7 o'clock. Creates a perfect oval and fit for my thumb on all my equipment including plastic.

I quit buying polyester spare balls because I've seen too many of them develop cracks around the thumb area.  I think it's because they are harder, and thus can get brittle over time. 
I have used a urethane groove for at least the past 10 years and it's as close to indestructible as there is. 

I agree with notClay about using urethane for spare balls, but I prefer the Storm Mix (Hybrid polyester/urethane). Also very indestructible, but skids like a polyester.  I went from Switch grips to slugs on my strike balls.  The cover expands and contracts at a different rate than the core/filler, so it sometimes leaves a ridge that can cause problems on my release, so I always use a slug.   On a side note, I like to take 3 strike balls when I fly, so I learned how to throw my strike ball straight, eliminating the need for spare balls.

Wish I could get my wrist to lay flat enough to do that. Plastic goes where I go

I learned it by trying to throw a backup ball at the 10 pin.  My wrist flops back and the ball goes straight.  Key is taking the fingers out of the ball and rolling up the back without turning the balls.

I agree with notClay about using urethane for spare balls, but I prefer the Storm Mix (Hybrid polyester/urethane). Also very indestructible, but skids like a polyester.  I went from Switch grips to slugs on my strike balls.  The cover expands and contracts at a different rate than the core/filler, so it sometimes leaves a ridge that can cause problems on my release, so I always use a slug.   On a side note, I like to take 3 strike balls when I fly, so I learned how to throw my strike ball straight, eliminating the need for spare balls.

 
Ditto on the Mix. It reacts almost the same as plastic, but the material around the thumb hole doesn't crack. When I used plastic, I got tired of the thumb hole repair every 6-8 months. The Mix is the Fix.....

Quote from: ignitebowling on September 16, 2017, 10:08:56 PM

Ive not used thumb inserts for two years now and it's been great. My thumb systems is a 15/16" thumb hole with 2 pieces of tape at 2 o'clock  and then 3-4 pieces of tape at 7 o'clock. Creates a perfect oval and fit for my thumb on all my equipment including plastic.

Do you use slugs/IT on your other balls?

No.  None of my equipment has thumb slugs or finger inserts.