I think the the emotional impact is important: preparing a return is still burdensome, even if not as bad as it could be, and that adds to and reinforces the pile of negative emotions associated with taxation.
I've tried telling people to save even just $50/mo. until they've built up a three month cushion, and they insist they can't afford that. You wanna try convincing them to also set aside a few hundred bucks a month for taxes?
My guess is that relatively few people, of those who currently prepare their own returns, would invest as much time and effort into verifying an IRS-prepared return as they do in preparing one. I would also guess that a majority of swing voters prepare their own returns (I know, that's a fair amount of supposition on my part...)