I'll be honest, I was immediately on-board with Ben Affleck as Batman. I was alongside a major minority that thought he was a great choice all around.
1. He's a talented director, and can lend his expertise to Snyder. Heck, it's probably through his involvement that they got Chris Terrio to do rewrites on the script.
2. Contrary to popular belief, he's a solid actor, and given his love for Batman, he'll look to get the character's mannerisms down better than any live actor before.
3. Given his huge name recognition, people will immediately discern him from Christian Bale, and get the idea that this is not Nolan's Batman.
4. And my favorite reason -