I am having a cast iron shower receptor installed. I asked if a waterproof membrane will be put down first and contractor said it was not necessary. Doesn't seem right.

Since there is no floor tile or grouting to leak, a membrane is not necessary. Fiberglass and composite receptors are not installed with a liner. May I ask why Cast Iron and not composite? CI costs 2-3 times more.

Yeah! I want to know why you chose a cast-iron base. Back to you, Tom

I chose the cast iron because I definitely didn't want a tile floor. Not the greatest housekeeper either so thought cleaning might be easier. Are there drawbacks other than cost?

Other than cost , weight, chipping and rust issues? None. Unless this is going in an upscale house, use a composite receptor. They won't chip or rust. If you drop something glass on a CI receptor, it will break and probably chip and then you have a problem.