Personally, I wouldn't ride on the track without one. I wear mine on the street as well. A good friend of mine hit a guardrail, sliding into it backwards - broke his hip, shattered legs, and many other injuries. Had to learn to walk again. He would have died without the back protector. Docs knew this because in the days after the crash, he developed black and blue bruising on his back in the exact shape of the back protector.
That is really what it's all about - taking the energy of a concentrated impact, and slowing it down (through varying density material) and distributing it across a broader area, while protecting more vulnerable areas.
Bite the bullet and buy a good protector. Then make up your mind that you don't ride without it. It will take a couple laps to get used to it, then it's not there anymore. If you can't afford a good one, skip your next trackday to buy it. It's that important, IMO.
I also recommend a mouthguard. It's a buck for an over-the-counter guard, or more if you want custom from your dentist. Again, following a buddies crash, where the chinbar of the helmet was toast, a doc said "I don't know how you still have all your teeth"...the rider was wearing a guard, but EMT's had removed it before the doc saw him.