H-Bridge Adjustable Slew Rate: Why is a decoupling capacitor needed?
You need to read the application notes.

Starting with page 39, it explains how different values of R11 influence the slew rate.

C2 is mentioned a few times, and seems to be non-critical - but it seems you really do need it.  

The most important comment about C2 is that it and R11 need to be as physically close to the chip as possible and that the ground connection needs to be very short as well.

I would assume that C2 is there to keep the voltage on R11 steady.  That voltage sets your slew rate, so variations would cause the slew rate to vary - maybe pushing you into territory where you cause too much interference.