And on top of that, considering how Driver chose to play Ben for the short period he had - Ben would have been pretty fun to bounce off Poe and Finn, too. Finn especially, I think.
Plus you then get around the need to change Poe into “Han Zero Sugar” as a character, because the snippy smart-ass dangerous hero role is now being filled by repentant Ben, and you don’t have to hard-yank the steering wheel on Poe over towards that end of the character spectrum.