For marathon runners, it's the final variable that becomes huge, and you just won't improve that without a solid base of distance running. Even 5k runners, for whom VO2 max and interval work is crucial, benefit greatly from one long run per week (usually around 20km if they're serious).
It might surprise you that there's been a general trend in cross-country at the collegiate level towards slower 90 minute daily runs instead of the faster 60 minute ones so popular a decade or two ago. http://runningtimes.com/Print.aspx?articleID=15744
You can lose weight with no cardio at all. However there is a veritable mountain of research that suggests that an hour of cardio per day is good for you. If you can't get that much, some is better than nothing. It doesn't have to be running all the time, either. Playing with your kids or going on hikes will do good for your health even if it doesn't single-handedly take care of your weight problems.
Running just happens to be an exercise that allows a particularly high HR and lactate threshold compared to others such as biking, rowing or hiking.
The reason for all of that is diet is the biggest factor BY FAR for weight loss and for most obese people, no amount of cardio will fix their diet. Cardio is not bad by any means, and to achieve true fitness you need strength and conditioning, but for most people trying to lose weight in a busy schedule, its place isnt as important as most assume
Cardio really should be a minimum of an hour per day, at least 5 days a week if weight control is the goal
First of all, who the hell has time to do a minimum of five hours of cardio per week? Some people have families or businesses to run. Getting a runner's high may be great for you, but you in no way need to spend an hour doing just cardio for effective weight loss.
My reasoning? The better your body performs, the quicker you see changes in it. Doing just an hour of cardio a day will not create a body at peak performance. You might be able to run a marathon, but you also might not be able to do a push-up. This is not your whole body working at peak performance. So you're missing out on vital conditioning by using all that time for cardio.
On top of that, you can get similar gains in endurance and cardiovascular fitness by doing a variety of high intensity workouts, and they need not last longer than 30 minutes (IMO) if done at top speed. I have no way to prove this other than the theory that lots of muscles trained to perform both slowly and quickly and at intervals of rest and high performance is going to have an overall greater conditioning effect than an hour of the same muscles doing the same thing at approximately the same pace.
I'll leave the meatheads to expound the virtues of resistance/weight training on strengthening the muscles needed to do such a demanding cardio workout in the first place.