I think that the distinction makes the code clearer to read. LET tells a future reader that they don't need to bother scanning each variable binding for parent

I think that the distinction makes the code clearer to read. LET tells a future reader that they don't need to bother scanning each variable binding for parent variables, whereas LET* tells them that they do. Seems like the same logic behind having a WHEN and UNLESS as opposed to just an IF, the former meaning that one needn't search for an 'else' block. The LISP family encourage good style.