Oddly, the JM may have better survivability on a typical battlefield, because the potential for delivering damage compared to armor is actually higher for the RFL, making it a priority target to remove that firepower from the field as quickly as possible. The JM's lower damage output means it's less of a threat, even though it's easier to remove, so it may not get shot at if there are better targets.
The problem with the armor of mediocrity is that I look at armor levels for the initial long range shots. Not many will connect, so I want the ones that do to have the best chance to hurt. The weak armor, low TMM, and the fact it is a long range duelist moves it up my target list. A PPC can take a fresh arm, blow off the armor, and be half way through the structure in a single hit. The Rifleman, with shorter range guns and heat issues can wait, a short bit. There is no safe crit to the arms, you have both cannons, the upper arm (+1 to hit if nailed) or shoulder (+4 for same). And if you punch into side torsos only two things are waiting there, a medium laser and a ton of AC/5 ammo. For all the potential mayhem of the Rifleman's CT ammo bin, at least there are plenty of other targets and the thickest armor in the way.
Too many ways to cripple a Jagermech not to toss an early salvo it's way in hopes of laming it for a couple light to finish off later.