I had a Coach suede swingpack that one SA told me to guard with a special spray for suede. I did as she suggested and the bag picked up every little piece of dirt. Soon my lilac bag was gray. I took it to a different Coach store and they gave me a new one. The SA at that store said never to do anything to Coach suede - it should do better on its own. So, I guess it depends on the bag. You might check with the company/designer.
I am so tempted to buy the Shining Monkey and test it out on my Gucci. I try not to bring my nicer bags out in the rain but just wanted to have some protection in case I spill water or I'm caught in the rain.