Well, as a fellow penmaker, I'll give you my opinion.
Whilst polyester is indeed easy and cheap to make, I personally wouldn't use if for my pens. I would never feel comfortable sending a pen out, knowing that if it happened to be dropped on a hard floor it could shatter, or if was accidentally bent a little too much it could snap. I know that any pen could break accidentally, but the more I can minimise the risk, the better. My customers are valuable to me, and I wouldn't want to ruin my relationship with them over trying to cut corners (or costs) through using an inferior resin.
Custom penholders aren't cheap, and the least we can do, as penmakers, is invest in the proper equipment to make pens which are worth the price tag.