That's correct. If you are already S-OFF, flashing the RUU does not set you back to S-ON and you don't have to relock the bootloader before running it. The only way to set S-ON is via the "fastboot oem writesecureflag" command which should not be issued unless you are sure that you have a signed stock bootloader installed.
For S-OFF users, as long as a nandroid is done before the RUU is flashed, the only things that are required afterwards are to flash a custom bootloader and recovery, then restore the nandroid and you will lose no settings. I agree that if everything is running fine and you are S-OFF there is probably no need to flash the RUU, but if you are having problems the whole process including taking the OTA (if you can see it) shouldn't take more than an hour.
For S-ON users, I think it's a good way to make sure everything is fully stock and in sync before doing the wire trick, but many have done it successfully with custom ROM's and recoveries installed.
ramjet73