He is full of crap. Assistant instructors will have a few less costs(insurance), but the AI cert really means nothing other than you can assist an instructor during a NAUI class. Divemasters on the other hand are full fledged dive pro's, that are required to carry liability insurance, and are far more useful in the world of diving. Sounds to me like the shop wants to use you up for a while before signing off on your Instructor ticket, its the unfortunate reality in this buisiness and is almost slavery. They know that they can get you to basically teach classes for them(in the presence of an instructor), and work for free for a while before they do an ITC for you.
Just my opinion.