I've used slaughters a few times in pairs to great effect. Theyre one of the better assault walkers in the game. Cheap, well armoured, maneuverable (deepstrike and fleet), and they throw out a massive numbers of attacks that will mulch pretty much anything they get stuck in with. A squadron of 3 seems a bit overkill to me, as two will handily kill anything but the largest hoards, and has the added durability of being able to fight back-to-back, effectively increasing their AV to side/rear shots, and also repositioning to mitigate hullpoint stripping.
As for the harpoons, theyre definitely worth while. As you pointed out, they more than make up for the lost attack through super-rage and rampage, and 5pts for a short range krak-missile launcher is already a pretty good deal, but add in the fact that theyre basically the only thing that can allow for 'assault' after arriving from deepstrike, and theyre golden. Drop them next to a vehicle or MC off of an icon, and if they don't kill it with pens, theyve got a good chance of dragging it in, which then frees you up next turn to go smashing around other things.
I'm not a huge fan of the forgeworld model though, its got wierd little leggies. Here's the kitbashed pair I made.