Cgrand, I did see your example of redefining cond after most of the language is defined, but since I was able to figure out how to do it in the proper place, that makes the :let bindings available for users of cond downstream and avoids any unforeseen complications that might come from rebinding.
As for your other point, I think it is highly improbable that someone would have used :let in the :else position. However I can imagine someone intentionally using something like :true or :default. I think the idea of warning for other keywords is actually more likely to cause complications than the unlikely problem it is meant to solve.
I did resubmit the patch with the docstring restored. Thanks for pointing out that problem. I'm excited about this patch – I use :let bindings within the cond in my own code all the time. Thanks again for the blog post that started me on that path.