There's a priming arm on my Rockchucker that's been there for a long while, back into the 70's anyway. Ain't broke, no reason to fix it.
I have them on several of my presses as well. It's probably me, but I have found it easier to use an attachment on the press. Not sure why, but I suspect that it's easier to get the primer onto the Lee than the press priming arm. It would be individual tastes.