FWIW, I found them useful packed with rocket launcher 10s. Put that in the hands of a Spider which can jump all over the place and can carry 6 RL-10s, and suddenly you have a really nasty backstabber. The best part is that the RL-10s are like twice the range of the Spider's normal weapons.
And I've used some Patron Security Mechs (XTRO:RetroTech) in an urban ambush scenario, equipped with a similar weapon. Really nice for them to step out, fire off all three RL-10s and then fall back into cover. Nice to make vehicles (and even light 'Mechs) think twice.
Other than that, the handheld weapon (HHW) isn't really effective. You can pack more weapons and heat sinks in a 'Mech's skeleton or vehicle chassis, and note that the 'mechs I mentioned earlier don't have a lot of torso/arm mounted weapons. If you use a HHW on a BattleMaster, Thunderbolt, or Warlord for example, you actually lose a great deal of firepower since the torso and arm mounted weapons can't be fired while carrying a HHW. (Though again, for ambushes, maybe.)
The only other situation where I can think of it being useful would be in an urban clearing scenario. If you lack units that are effective anti-infantry units, you could probably bang together a HHW with a machine gun or vehicle flamer pretty quickly. You'd just be limiting one of your units. (Maybe give it to a Panther?)
Actually I wonder if the Capellan Confederation would whip up some HHW's devoted to laying/destroying minefields. Something to experiment with.