Although I hate cutting ANY leaves at all because they are beneficial to the plant, depending on the strain and size, I will sometimes cut the lower branches for the exact reasons you stated.
I've had mold and gnat problems in the past, pretty bad at times. Cutting the lower branches has definitely helped eliminate those problems. The base of your plant is very stuffy and full of leaves, a place where mold and bugs thrive. By cutting those branches, you allow for more air circulation, which will deter those pests. And like it was said earlier, cutting the lower leaves and branches will allow for more growth concentration in the canopy. In my experience, those lower branches typically produce tiny "popcorn" nugs that are a pain in the ass to manicure and aren't really worth it. The buds don't usally get enough light to grow big because the canopy is intercepting most of the light.
However, these mold and bug problems usually only occur when you have a group of plants in a confined area. In your case, your only growing one plant. Therefore, I probably wouldn't cut the lower branches because they are getting suficient light because it doesn't have to compete with other plants.
By the way, what kind of light are you using and how far from the plants? The plant looks a little strecthy...if you can get the light closer to the plant without burning it, I would definitely do that.
