Thank you for writing up such a clear explanation.
There are definitely safety benefits to using the simplest form. But I would prefer to use the most general form (withs) because I won't need to worry about using the most idiomatic form or to switch from one form to another as the program changes. It frees me to worry about other things.
Being able to tell simpler parts from more complex parts at a single glance isn't as much of a concern for me. I noticed I don't go back to even read code of programs that didn't solve a big enough problem. The programs reach a dead end and don't develop further. There are more important things to worry about when writing a program.
Also note what let, with, and withs offer could be provided in other languages too. Them being implemented as macros in Arc is incidental. You're not giving up the benefits of lisp by not using them.