Is it just faster? More persuasive? Better at coming up with novel arguments?
Speed is great from a client perspective, but not as important as accuracy on important stuff. I haven't seen LLMs produce arguments that are more persuasive than those crafted by good lawyers. Finding the "perfect case" in Westlaw is helpful, but rarely the deciding factor.
I could see asking an LLM to reproduce your work AFTER you've done a first pass, to see if you missed anything -- that seems like a real value add. But I truly can't understand how not using it would be "malpractice." In fact, I still think committing malpractice would be MUCH more likely with AI. (Now if by "malpractice", you meant pissing off clients with expensive bills, that's a different story...)