I actually did it this past weekend and it worked pretty well. It rained really hard over night and although some water saturated the canopy and did come through to the tent, the tent fly was a good second layer to prevent a good soaking. I was bone dry inside and pretty happy with the result. I don;t think it will replace a good tarp, but I pitched my tent in a clearing as it was the only viable spot. There was no way for me to rig a tarp unless I just draped it over the tent and weighed it down with rocks. My buddy convinced me to give the canopy a try and as I said, it actually worked fairly well. I don't think it would be good for more than an over night shower though unless I water-proofed the canopy itself. I may do that.
I did not have sides on the canopy as mine came with mesh sides and the mesh is actually a liability in the rain. Brush up against it and you're soaked!