There's only one place in the world I can name that anyone makes a point of visiting that is famous mostly (or solely) for gambling, and that's Las Vegas. I don't mean that people from Ontario don't take bus trips to Niagara Falls or Casino Rama, but no one saves up to take a trip to these places as some kind of "you have to see it before you die" trip. People into gambling no doubt look for opportunities to visit places with casinos, but if you're talking about driving tourism in numbers worth discussing you have to attract non-gamblers to come to your casino. I'm not a gambler, and I've been to Vegas (once, and that was enough). I actually didn't even gamble when I was there. Of course I'm an anecdote, but the evidence from other cities not known for gambling that add a casino (like Montreal) backs this. A casino is a regional big city attracts people who live a reasonable drive away, but no one visits Montreal who otherwise would not just because it has a casino. I love Montreal and am happy to visit it when I get the chance, but until this casino debate hit Toronto, I wasn't even aware it had one. I don't know anyone who has gone to Montreal for the casino.
A small place like Rama, Ontario will of course benefit enormously from increased regionally local tourism to an attraction like a casino. Toronto already has all kinds of things to draw people from across the GTA and southern Ontario to visit - major broadway productions, pro-sports teams, international music stars doing concerts, high end shopping, museums, art galleries and on and on. We're a big city and I find it beyond difficult to believe we'd even notice the addition of a few more Ontarians visiting Toronto for the casino who otherwise don't come to the city for those other things. In fact, there's lots of reason to think those other things will suffer as people who used to come for them, now redirect their leisure dollars to the casino instead of a play or concert. Zero sum stuff.
There is one place I can think of that has attempted to make itself into another Vegas, and that's New Jersey's Atlantic City. Multiple casinos, a beautiful boardwalk by the ocean, and yet, can anyone claim it has worked? Again, I don't know anyone who's been to the place. After all, if you're having a Stag and want to do the "big casino town" thing, why would you travel to 2nd place, when a flight to Vegas will be quite comparable?
Enough. I really shouldn't be even stooping to argue this as if it was a serious proposition. Those making the claim that a casino (even some kind of "mega-casino") would drive real tourism have the onus on them to supply the proof. Whatever arguments we're going to have about having a casino in Toronto (I oppose, pending some convincing proof that enabling more destructive gambling addictions has sufficient upside to be worth it) should be made on non-preposterous arguments that assume a modest regional tourism boost at best. Maybe having a private developer rebuild Ontario Place or the Ex is "worth it" but that's where the debate should be. Anyone pushing the big-tourism boost line should be disregarded as disingenuous or delusional.