i'm using my lower brand bag, I just cannot be able to cought redhanded in a fake bag, hehe. Plus, once you carry the fake, people will questioning your other bag.
I certainly wouldn't use a fake just because it's raining. There are so many other options besides using a fake bag. I usually don't let the rain stop me from carrying a nice bag because most of my bags are able to withstand the elements. If I'm unsure, I'll put the bag under my coat or shirt and quickly run for cover. I just can't get into carrying a fake.
LeSportsacs are great for rainy weather! Get a nice colorful pattern or a really cute Tokidoki print and it'll brighten up the day a bit too. AND they're washable.
I also have a collection of older Coach Sheridan bags made of all-weather pebbled leather and carry those when it's raining or snowing. Nothing soaks through them.
And carrying or even buying a fake is a no-no regardless of the circumstances!
What is the point? I honestly don't get it. On days where the weather is less than desireable, I simply carry a bag that I don't care about as much that I've had for awhile. If you don't have that, I personally think a $20 bag from Target would do just fine...