The more I read your posts, Frostiken, the more I become sure you lack even basic skills. Most problems you have with weapons or equipment comes not from those things being badly implemented but from your inability to use them properly. Seriously, man, play MWLL regularly for at least for a month or so, get some kills, be at least GC in every game you play and then rethink your ideas.
JJs are good for:
* Getting to otherwise inaccessible places.
* Overcoming obstacles.
* Quickly getting behind cover.
* Escaping mech going critical nearby.
* Poptarting -- yes it's more than usable. Ask all those people that I and many other SCat pilots killed that way. Or Cougar A pilots. Or MK II arrow spammers. Or even NCat A boaters.
* Evading enemy fire. Most of missiles could be evaded. FBombs could be evaded. LT could be evaded. Even hitscan weapons, like lasers, could be evaded by jumping out of enemy's reticule and ruining enemy's aim. Insta hit don't mean insta damage, BTW. Lasers need about a second to fully discharge. Yes, your mech stops after landing, but as a lot of weapons need considerable amount of time to recharge, you have enough time to reaccelerate, especially if you have MASC.
* Close combat maneuvering. While this type of maneuvers is quite hard to do properly, it is possible to use them for your advantage. It's really a countless times one single jump was the decisive action between life and death. Suddenly appearing where your enemy do not expect you, jumping out of the way of advancing enemy to let your teammate to get a clear shot, getting to enemy's blind spot quickly: all of this is extremely useful and could be done effectively with some training.
Of course, JJs are not very friendly for a new pilot to use, but with some skills they become a powerful tool for a mechwarrior.
And, BTW, you cannot get a mech kill with MGs, they don't do any damage to heavy armored targets, like mech or tanks.