The only reason to have one is if the races are put on by a sanctioning body such as SCCA. Then follow their rules. Usually if it doesn't say you can then you can't! That being said there is no reason to have one. The only classes that SCCA might require them to be there is SS, IT, & ST. There might be one other class but all of the Prod, GT, & Formula classes do not require one.
I've worked on GT cars and we used chocks to keep the car from rolling.
My current track car does not have a E-brake. It's a PIAT to change pads, which you often do on a race car, and the cables on my car were old and i didn't feel like spending money on new ones since they don't make the car faster. As a safety issue it's a non-starter. If you ever has a brake failure on track the last thing I would do is pull the Ebrake. The rear brakes will either barely slow the car down meaning you'll only hit something with a bit less force or they'll lock up the rear wheels and you end up going off track backwards and become a passenger with no control on where the car will go or what it wll hit. I prefer to be able to see where I'm going, what I might hit, and be able to steer away from it if possible.