I flex a lot inside and outside of the gym. It might look weird to some people, but I don't go to the gym to try to impress people. I like flexing between sets because it pumps more blood into the muscles I'm working, which gives me a better mind muscle connection. Flexing in the mirror lets you study yourself. Also, I think it improves definition in the short term, to an extent, if you're constantly practicing your flexing.
Flexing is essentially an isometric exercise, right? You're contracting the muscle forcefully, but not over a full range of motion. Still, muscle fibers are contracting. Think about this: squats and deadlifts are two of the most beneficial exercises for gaining muscle mass because so many muscles become activated to achieve the lift. A lot of the muscles being worked are only being worked isometrically to help stabilize the body. So doing squats and deadlifts involve you doing a lot of isometrics. What I'm trying to get at is, isometrics are beneficial to any exercise regiment.
Another thing... If you're going to be doing a lot of flexing, make sure you're doing a lot of stretching. Muscle fibers tend to shorten after contractions, over time. To oppose this happening, stretch the muscle after periods of contractions. This is why static stretching is so useful after a workout. All during the workout your muscles have been contracting to the maximum, so afterwards you want to stretch them back out.
That's just my opinion on flexing and stretching. It's been working well for me so far. And though I can't cite anything, I know there is some science behind it.
