It would have given his character depth. It would have given Harley depth. It might have set up there relationship better and kept it closer to what it has been since Harley’s inception in ‘91, instead of what it is in the film.
I liked Margot Robbie’s performance, but her Harley annoyed me. In part because she couldn’t decide whether to use the accent or not, but mostly because she didn’t come off as Harley. Harley’s incredibly intelligent and skillful despite her madness, and knows that she’s in a very abusive and toxic relationship but can’t help herself. The Harley in Suicide Squad never gave me that vibe, she didn’t come off as intelligent or mad, just wacky for the sake of being wacky and it portrayed her relationship with Joker as healthy and reciprocated, albeit in an unorthodox manner.
Most of the characters could have used more depth, or reason to be there, but that’s what happens when you have 5 characters who split maybe a dozen lines between them that are still part of the principal cast.