An unfortunate and redundant waste of TRO and RS space. We've had a lightly armored, 3/5 tank that can throw 60 LRMs from four turreted launchers with a couple MGs up front since the Succession Wars. It's called the Partisan (LRM). And the Partisan (LRM) has an ICE engine and is tracked, so it's cheaper while being more mobile. The JES III has not only failed to advance the few-hundred-years-old state-of-the-art in mechanized missile support, it's actually gone slightly backwards from there.
There are considerations that work in the JES III's favor. Lower tonnage means being able to cross a larger variety of bridges, less crew, less maintenance for refueling. The JES III also has the unfair advantages of quirks like Easy to Maintain that the Partisan does not by simple virtue of being too old to have Quirks, Improved targeting (long range) isn't bad, either. It opens up the entire 8-21 span for the same to-hit range penalty.
For a national military the JES III makes sense. Pay for it up front, save on the back end as time goes on. In an era of peace the costs of fuel, crew and techs eats away at the cost advantages of a low initial purchase.