Why don't planes use pulsejets?
2 reasons really:


Airplanes need most of their thrust right when their airspeed is low. (to get more airspeed at takeoff and go-around). 
The maximum airspeed in cruise is not dictated by how much thrust the engines can throw out. Instead it's dictated by the aerodynamic forces on the airframe. They cruise right below the speed of sound where the rules of aerodynamics change drastically. Going too fast would rip an airplane apart.