What decides if you rebuild a carb?

I got a Holley 750 double pumper for free from a guy who was just trying to get out of wrenching. It's the newer style with plastic pieces on it. I opened the floats and both were clean, floats moved easily. My cousin has been on me to rebuild the carb, but an engine builder I know said to just run it.

What designates a rebuild? Is it in my best interest to just go ahead and rebuild this carb?