I've learned this the hard way with interstitial and banner ads throughout releasing a dozen or so apps. You really need a rather large user base to make them even a bit worthwhile to a developer. Scratching out pennies will have a huge downside if it lowers the experience of the app to your whole userbase. It's an art form not to make the user feel like a product. Everyone thinks they know how but really so, so few do, myself included. I've also put down many apps because of ads. It's stopped one of apps from moving as quickly as it should. I've since moved away entirely and incorporated IAP that unlocks slightly more features or a straight up price tag from the get-go.

Anecdotal, but I believe this led to much less moaning and groaning from users, higher ratings, and a better overall experience (and yes, better returns)!