A pit does require a through the roof vent. The air in the pit has to leave as the pit fills with waste and it has to come in as the pit is pumped out. The traps of fixtures also have to be protected by venting. This venting of the pit cannot be accomplished by an AAV (Air Admittance Valve) as it is a one way valve. It is good that your plumber insists on having a through the roof vent.
How this vent is run can have a lot of variation. Local codes do have bearing on what is allowed. In some areas it may be required to be independent of other vents, while in others it may tie into existing vents. In any case the vent must run pitching back to the pit so that any condensation in the vent or, rain falling in the vent will drain back to the pit and the vent will drain dry. This is a case of there may be many ways to do the same thing. One thing I try to consider is finding the least intrusive way possible to run this vent. You may want to discuss this with your plumber and have him explain his reasoning. If you don't care for his way of doing it seek another opinion. The imortant thing here is you need that vent and the work should be done in accordance with your local code. This work should be permitted and inspected.
Properly installed you will have a good running system with no smells. Improperly installed you will refer to it as the "Stank Vat."