It feels sleazy to me to draw users with a certain level of page quality, while planning to reduce that quality later after they get in the habit of using it and recommend it to their friends.
you don't have to promise not to run ads. but ads are a drag on your acquisition of users, and the quality of your site improves with the number of active users -- so are you doing a disservice to your early users by running ads?
I think you're just rationalizing here. My response would be: sleazy it is, but if it's what it takes to make it succeed, it's your choice to make. Besides, it's not sure at all you'll eventually use ads; they are not the only way to monetize a site. So why move there before it's really worth it?
Web page quality reduction caused by ads = (Some constant)
So, you plot that out and find out when the money derived exceeds the damage. Requires guesswork, of course.