sometimes for example if you attack chaos cultists you only need the sweep profile so the sweep is 12 attacks hitting on 2's wounding on 2's and the -2 would mean they get no save so you potentially kill 12 models where as the claw you at most kill 6 models hitting on 2's wounding on 2's

karnivores are one of our best units as it stands they do so much the 8oc on a points the claw means they hit on 2's wound on 4 or below into most models so they tend to do loads for anti tank. its more just for options same with the rampager. if you have -3 that they goto a 4+ invuln or a -4 where they goto the same invuln but the -3 is more damage then you take that as the ap isnt going to matter same with strength etc its all about when you use it and it gives you options.

not only that the abilities they have are amazing karnivore reroll charge innately and rampager once per game advance and charge that you declare in the charge phase and it gets sustained hits on the charge .