No one, and I mean no one who was ever good at tennis in the history of the sport hit the ball without spin. Spin gives you control and adds difficulty for your opponent by making timing and tracking more difficult. I've played against and with people who hit the ball truly flat at the club level, those are like knuckle balls, the ball literally has no arc, it just goes whatever way the wind blows it, and has almost no rotation on it.
By flat you must mean without shape(as the Aussies call it). All players hit with spin, so when you say someone is a flat hitter this has nothing to do with the level of spin, the only thing that varies is the arc the ball takes when it's hit. Infact to flatten the ball out requires alot of spin for control. The level of arc on a ball has to do with the area and angle of the racquet face when it strikes the ball, this gives the ball more severe or less severe arcs. Today the modern player is capable of hitting many spins and arcs(flat or loopy). It is only the truly one dimensional player who hits the ball flat(without arc) all the time. More arc makes the shot higher percentage(room to get over the net, or take pace off the ball and still get it back deep so you can have more time to return to the center of the court) thus making loopy shots a valuable shot on defense or neutral play. Then you have players like a Nadal who hit those loopy shots very fast, making it an offensive weapon. Of course this is over simplified but, simply put a flat shot is an offensive shot in most cases used to take time away from an opponent by trading percentage for speed on the ball. A player is limited if they are unable to hit the ball in a variety of ways...
). Plus, adding spin brings more variety and quality into the game anyway.
And by slowing down all possible surfaces thesedays, flat shot isn't such a weapon anymore. :shrug: Which is a pity, of course.
