Why can't they run these marathons at night? Is it time to ban these things, or at least limit them to one per year?
I presume you are just being sarcastic with that first question.
When demanding a ban or limitation, on what criteria do you use?
I presume your issue isn't that it happens to be thousands of runners, but that the roads are closed, regardless of the reason. You do realize that in a major city like Toronto there are dozens of such road closures that have a significant impact on traffic?
All kinds of parades, Santa Claus, Pride, Labour Day, etc block off key streets for hours, usually in much higher demand periods than the two marathons. Then there is road work (summer weekend DVP closures?)
Both marathons have a cut-off of 12 minutes per mile pace (they re-open the roads at a rolling rate of 12 minutes/mile or 5 miles/hour). This means other than the immediate vicinity around the finish, everything is clear 6 hours after the start.
This weekend's Goodlife Toronto marathon started at 9am (half started at 8am), so most of Yonge was clear by 11, University by mid-afternoon. On a Sunday morning, about the lowest traffic demand you are going to find.
Waterfront marathon started at 7:30 (and had less city bisecting impact as it was primarily run east and west near the lake, only coming up north as far as just north of Queen on Bay). That means early afternoon for no east-west impacts.
As for positives, as others have mentioned, both events not only get thousands of people active, but bring in hundreds (if not thousands) of out of towners. Total participants through all events are on the order of 20,000 for Waterfront and 15,000 for Goodlife.
Have a look at the hometowns of those in each of the full and half marathons:
Goodlife Toronto:
http://results.sportstats.ca/res2009/torontom.htm (full)
http://results.sportstats.ca/res2009/torontoh.htm (half)
Waterfront:
http://results.sportstats.ca/res2009/scotiam.htm (full)
http://results.sportstats.ca/res2009/scotiah.htm (half)
Not only does that bring in dollars, but having such major events is a fundamental part of what makes a city a city and not just a large number of people who happen to live and work in close proximity. Just like parades, street festivals and major exhibitions.