To me, a JM or RFL might as well have a giant bull's eye painted on them. As thin as the armor is, it takes very little effort to put a significant amount of opposing firepower out of the fight quickly, and I will often do so even instead of a marginally easier shot at something better armored. Basically, their "firepower-to-armor ratio" is the pits.
Shooting at a Thunderbolt or Awesome early in a battle is almost a waste of a shot, because it's going to be there close to forever, no matter how much fire you pour into it. Ditto for the Annihilator, that's going to keep coming at you (but slowly) long after your ammo bins run dry. Kill the easy stuff, to reduce the amount of incoming fire ASAP, and THEN go after the hard targets that are left.
Then there are those quirky "well armored, but STILL vulnerable" designs, such as the Crusader/Roman Candle (CRD = Candle, Roman, Distributed fuse), the Trebuchet (8 turns of LRM ammo, and only 5 turns of armor), Marauder (near guaranteed TAC to the Left Torso), or Hunchback (4 points of back armor over either the AC or the ammo for it, and even a basic Locust carries a ML which does 5 points).