When your strike ball is plastic, it doesn't exactly make a lot of sense to switch balls to another plastic ball for spares. However, on some of the old shows, they mentioned using a shinier ball for spares. But still, plastic is plastic, and when all your balls are plastic, you don't have much choice but to flatten it out.
As for opening a spot in the bag, I've found that a plastic ball is just as valuable as any other ball. I've used mine as my strike ball on several occasions, and plastic gives a look that reactives simply cannot.
SH