If I have a leak in my brake fluid reservoir I'd rather know about right away. Fixing paint is easier than fixing everything if your brake are mush and you can't stop.
You're assuming that leaks only happen at the reservoir, what about the caliper, lines, banjo bolts? Why don't you put a sock on your calipers? Your banjo bolts? Your brake lines?
Agreed.
The amount the seeps from the Master Cylinder is never enough to really splatter everywhere. If it is in a volume that is spattering everywhere on to paint, then you have way bigger issues than brake fluid messing up your paint.