Well, first, why so angry?
Second, as I said in my last post, regardless of whether they SHOULD or WOULD, I think it's wrong for people to say they COULDN'T. Obviously they could if they wanted to.
And if one really wants to get worked up about that comment from Diggle, then yes, the *existing seasons* of Arrow could take place before Man of Steel, but we could come back for the third season (probably the 2nd episode since presumably the first will be resolving some cliffhanger) and Ollie and his group could be gathered around a TV hearing reports about what has "just" happened in Metropolis. It's completely doable.
Now, if you want to shift the conversation over to why I think they SHOULD, I would say because Marvel is experiencing tremendous success with it, thus proving that people like it. It says "hey - do you like this? There's more of it over here." It's awesome when characters team up. The super-hero genre is the only place it really makes sense to do it. Do you know how badly I wanted to see Knight Rider team up with A-Team when I was a kid?
At the end of the day, the human mind wants things to be consistent.
Every excuse you make for why "Arrow" and "Man of Steel" can't exist together, from a setting point of view, could be asked about the comic-book versions, and yet they do just fine.