With EVOs prior to the HTC EVO 4G LTE, the main benefit of S-OFF was the ability to flash radios without reverting to stock and doing an over the air update. But even then, you were stuck going forward and could not revert to an older radio in case the newer one did not work so well. Luckily, Captain Throwback has delivered a method for the HTC EVO 4G LTE which circumvents the S-ON restrictions to recovery mode and allows any versions of the radios to be flashed directly from recovery.
The two notable times I can remember needing S-OFF to change radios were once on the OG EVO, when a radio update was eating my battery alive and I had to jump back a version, and more recently on the HTC EVO 3D, when I wanted to try a ROM that had a different baseband and had to go forward.
With S-ON/unlocked providing no functional differences to S-OFF, the only barrier to a completely free phone is the requirement to use HTCDev.com to unlock the bootloader. As HTCDev states, this may invalidate the warranty, and many people believe that once you’re on the list, getting warranty work could potentially be an issue. (This should not be any issue for Sprint users, as we know what the official Sprint repair policies are.)
There appears to be no performance benefit to flashing either of the radios available for the EVO 4G LTE, and with flashing radios you always have the potential for something to go screwy, so I’m going to hold off playing on radio flashing until such time as there is a reason to, but it’s nice to know the method is there when we eventually need it.