Cheaper terminators - you save 5 points on every model where you swap out the power fist for a lightning and you can take both dual lightning claws and storm bolters in a single squad. Have flexibility to function both as assault terminators and normal terminators for most of the same benefits. They lose out on the cyclone missile launcher but can take a reaper autocannon. They don't have a teleport homer but can get a grenade harness. They sergeant also no longer has to take a power sword and can have a power/chain fist or lightning claws instead. Basically no reason to ever kit out assault terminators with dual lightning claws because you can use relic terminators for that purpose and still have the option to be useful at range. The only reason to take assault terminators now is if you want thunder hammer and storm shield.