The Space Shuttle, especially when traveling at low supersonic and subsonic speeds, has the glide ratio of a crowbar. The trajectory is designed to ensure that the Shuttle arrives over the landing zone at a specific altitude and velocity (sometimes refered to as the "high key" point). Factoring in variables like high altitude winds, trajector errors, etc., the S turns allow them to precisely tune the trajectory to hit the "high key". If they tried to time the deorbit burn to precisely reach the high key point without any turns, then any unexpected change could cause the Shuttle to come up short. If anything, it's far better to arrive at the high key point with a little extra altitude and velocity. Given its subsonic lift-to-drag (or glide) ratio, the Shuttle has little trouble losing altitude or speed.