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.