However, if you put 15G down your "20G" path, you have no redundancy. In a cut, dropping 5G on the floor, causing 33% packet loss is not "up", it might as well be down.

If your redundancy solution is at Layer 3, you have to have the policies in place that you don't run much over 10G across your dual 10G links or you're back to effectively giving up all redundancy.

This can be a valid solution. Our company has some multimedia traffic that can be axed if a catastrophe befalls our tubes. Obviously we'd prefer not to drop (or ax, as it were) any type of traffic, but it's a monetary decision.