Originally Posted by
cyccommute
I disagree, as you well know. Yes, you should never use the small/small combination but you shouldn't use the big/big either. But with the chain as slack as El Gato27's, you risk chain bounce that can trap the chain in a chain stay or the chain can end up wrapped around the derailer and will end up tearing the derailer off the mount. In my experience, this kind of damage is far more likely and far more common than damaged caused by having the chain too short.
Well, the possibility of damage is there but it gives you ample warning. As soon as you shift into the small-small combination under these circumstances the noise and rattling you will know it doesn't feel or sound right and you will shift out of it promptly. Having the chain too short for big-big gives you no warning at all. As soon as you shift into it, the damage is done immediately.