Each time you pull the trigger two contacts make an electrical connection. Just before they make a connection an electrical arc will travel from one contact to the other pitting or charring the contacts. This happens each time you pull the trigger so eventually you'll burn out the contacts (assuming there is no mosfet installed). In other words semi is worse for your gun (don't worry this happens over several thousand cycles and IF you do actually burn through the contacts they are replaceable for about 35 bucks)
It's generally recommended that you install some sort of mosfet, whether it's a basic unit (20-40 bucks) or a multi function firing computer (trigger master etc.) You'll notice a slight increase in ROF and trigger response as power is directly transfered to the motor via the less resistive mosfet.