Water resistant does not mean water-proof!
From my experience, water resistant is not the same thing as water proof! And spring showers aren't the same as thunderstorms :) If you live in an area that rains a lot during the winter, I would rather invest in something that is water-proof, even if it is just a thin rain sheet. I am more concerned about keeping my horse dry, especially if the temp will dip below freezing at night or if there is a lot of wind. If he's dry, his winter coat should keep him warm enough
If you do get this blanket and have no shelter for your horse, I would not keep a wet blanket on horse. Be sure to soak it with some kind of water-proofing spray, like a Scotch Guard type thing, and check on the blanket the first time it rains.