I'm not sure if you know this but, the Grasshopper is not the actual vehicle that will make the soft landing after launching the payload. All it is doing is testing the procedures upon SpaceX will recover the Falcon 9 V1.1. All of this aside, the reason why parachutes would not work is for one they have been tried on the earlier launches and it never worked. The first stage of the Falcon 9 is also grossly heavier than an SRB. Also when the SRB's hit the water they become filled with salt water, an extremely corrosive mixture which is costly to clean and repair. Another reason to have the ability to have a launch vehicle which can return softly to the launch site has been shown with the shuttle. A quick turnaround between launches becomes possible. That, in theory, should drop launch prices as time passes.