1st) If its a free game no developer should have shame with putting ADs in their games. We work hard and deserve a little revenue for our work in making free entertianment for people. If your a developer never let anyone shame you into thinking you shouldn't put ADs.
3rd) There are other AD networks like Notifymob and Airpush that pay more. With Notifymob they will put the ADs in the game for you. With Airpush they will make a plugin for you but you need some coding experience.
Finally do not listen to people who complain about ADs. There was a recent war on that very subject online it was on facebook and another android forum. Developers stuck together and finally quieted anyone who complained about ADs. I wish all you success and I will post more marketing tips and ways to promote your android apps. I have a large android forum where you can post your game or apps for more exposure too.
I can't post links so just combine the http with the other part. Any questions you have feel free to ask.