The post in question was actually about conciseness as a metric, and the example used in his argument was exaggerated to make his point, but the form he created (the modified let) might actually be useful (although the last stage of his example was a little scary... maybe we shouldn't take it all the way).
I don't think mixing the two is good. I think the lack of implicit progn would be a Good Thing. The explicit progn makes non-functional code stand out, and it also makes the language a bit more uniform if no primitives have implicit progn. All other things being equal uniformity is good because it reduces surprises.
Unless there are a lot of people who feel like the implicit progn is an important part of "let" I'd prefer to not have it.