51 Chevy windshield install question

Going to put a one piece windshield into my 51 Chevy hardtop. I have read to use a non hardening silicon.the shop who is going to do the install said they can't use anything but their own current stuff which is a hard type.they said it also didn't need the silicon as it was only for water sealing.is this last part true?I never drive it in the rain.

 

I did mine without any sealer and it doesnt leak.