Just wanted to point out that within the boundaries of TV and movies, having characters cross over is extremely rare. We don't get to see Rambo meet the Terminator (although they did manage Freddy vs Jason). I'm not just talking about super-hero movies. And while I understand you might prefer they do their own thing, surely the fact that Marvel did it first wouldn't stop you from enjoying the end product? Like you would avoid a Justice League movie JUST BECAUSE it was copying what Marvel did?
And sure, DC could manage to screw it up, but the pretty much the foundation for being successful is looking at what another successful entity did and do the same thing as much as you can. That's like saying that one athlete shouldn't try to play his position like another very successful pro athlete. Do what works, make your money.
At the end of the day, for me, the good of tying it together outweighs the bad, and with most of the reasons people give why it wouldn't, there's usually some example in comics or in other franchises where they'd done something similar and been successful with it. I think DC isn't doing it because that extra bit of effort is more than they care to put into it. What they are doing is certainly the easier way of doing it, even if they are leaving money on the table.