Yeah overall swarms are about the worst choice in our army. They are really no good at all. Sure they can be made effective with spell combos, but wouldn't you rather buff the strength and toughness of your other units?
LM are strong enough that they can afford to carry the unit in most games and still be competitive so if you do like them for whatever reason, go for it. Just don't expect a lot. They can hold up or redirect a unit for a while, they can also work on flanks against skirmishers or fast cavalry. They could potentially be mage hunters (can't be challenged, get 2 in contact and you can drop 10 poison attacks on a mage in a unit!) but don't really expect a lot from them.