If you are a strong individual and the golf ball jumps off your club head and flies down the fairway with power and purpose, you may not need the extra power added by using flexible or regular shafts. If you have a swing speed faster than 100 mph, you likely need a stiff-shafted. A stiff-shafted club needs more of an effort to flex it, and it tends to result in straighter hits at high speeds.
These are the clubs that most golfers use. They will flex more easily than stiff-shafted clubs and will give a golfer with a swing speed between 90-100 mph a bit more distance than stiff shafts. The flex is not so severe that it will compromise the accuracy of the shot unless the golfer takes a wild swing to generate as much club head speed as possible.
Everybody likes to hit the ball for distance, but in the long run accuracy is the key to becoming a successful golfer. If you are constantly hitting your ball out of the trees or from some other hazard, golf will soon become a chore and you will count the minutes until you are done playing. So if you are debating whether to go with clubs that promote accuracy or distance, go with accuracy.