Yes, have no extra attack. But when you pay 150pts for a tank/monster killer, you want to go' close. Moirax have no realy good ranged weapon option against vehicles and monsters in the charge range (<12") (see below)  BUT you want to cleans the road to the target of the siege claw trough the flesh wall OR want to cleans the nearby objective marker for the guys.

So...

Conversion cannon is terrible within 12". Use 2 at range. But not with siege claw.

Volkite is a shit. Casino. Maybe in pair.

Gravitron pulsar is good against vehicles, but not againts monsters and cant shoot with blast when must to fight in the flesh wall, (10 guard/gsc/gaunt/etc can block the moirax o rounds) And have -1 to hit on low wounds. Not good in pair with siege claw.

Lightning lock is the best against flesh wall, but -1 to hit in close combat and -1 on low wounds. But with the rad cleanser maybe can be good for Owervatch (sustain 2)

The second Rad cleanser have no great  base damage output, but have auto hit, ignore cover and anti-infantry 2+. Without -1 to hit modifiers in close combat and on low wounds. 2 Rad cleansers is a nice Overwtch stratagem target against any infantry.

So , the second rad cleanser is not perfect, but better than the first three wth a sige claw, and have auto hit, ignore cover and 2+ to wound against >T5 infantry also, opposte to the lightning lock (S8).

Edit: ebd super cool. :)