I guess JJs are a matter of taste mostly.
I don't like getting close to my enemies, since that makes it easier for them to get into my side / back and exposes me to melee & DFA attacks. Which often results in repairs after the mission, since those put a lot of dmg into a single location. In Campaign / Carrier that's annoying.
But since I like to stay at range, I need to bring long range weapons, which are heavier, so I can't afford to bring JJs at the same time. Also at range you need the added mobility less anyway.
I'm also hesistant, since every point of heat you put into jumping can't be put into dmg. So unless you jump into your enemies back, you're likely losing out. But doing that usually opens up yourself to getting hit in the back as well... and we're back at "I don't like that in campaign mode."
If you want to have JJs I would suggest to avoid using any long range weapons. Go full ML + SRM + AC20. If your 'mech isn't in range to fire those, sprint or jump into range. Then use your JJs to stay behind or at least in the side of your enemies. Still I'm not convinced a 'mech as fast as the Firestarter has any need for JJs.