Originally Posted by xFullCourtTenniSx
More mass means more tiring over the long haul. Higher swingweight means more force is required to get it going in a swing. Basically you need a strong body.
And I'd add to that that you need solid technique. I think it's tough for hackers or sub-4.0 players to wield a heavier, head-light (more lead in or on the handle than on the hoop) frame. But once you have solid mechanics and timing and you're using your legs and core for strength and balance coupled with a loose arm and hand for racquet-head speed, using a heavier, head-light frame is a big advantage both for your game and your physical well-being (particularly your arm).