They are both necessary and both safe. The purpose of the primer/cleaner is two fold and somewhat misunderstood. It cleans, obviously...but it also primes in a way that allows the cement to weld the two pieces into one. The primer SOFTENS the PVC surface. Once the cement is applied, the softening of the two surfaces allows the cement to weld these pieces together. They in fact, weld into one piece, if done properly. To do this properly, the PVC primer must still be wet when the cement is applied. After a brief curing time, it is all reef safe.
Used to work for my Ex Father in Law as a plumber. PVC Primer and Cement are safe for drinking from within 15 minutes of application. In reef systems, I've allowed as little as 20 minutes with NO ill effect.