You don't need to prestige it to get it gold or diamond.
The only things you get from weapon prestige is a weapon Calling Card, the access to another weapon Calling Card after you get all the camos, emblem, and clan tag. You do re-earn the XP for prestiging a gun so if you only use one weapon you can essentialyl earn the weapon XP 3 times.
Outside of that though, no there is no real reason to prestige the gun. For instance, I leveled the Ballista all the way up to get the Iron Sight. I will never prestige that gun as I hate sniping and only wanted the Iron Sight. No point in prestiging that gun to me. --- "Cover your receptors Perceptor." Driver picks the music. Shotgun shuts his cakehole.