Free apps eat up your phone battery just sending ads. Up to 75 per cent of the energy used by free versions of Android apps is spent serving up ads or tracking and uploading user data.

http://www.newscientist.com/article/mg21328566.400-free-apps-eat-up-your-phone-battery-just-sending-ads.html

I always buy paid apps anyway because...well they're like 69p and I want to support the devs.

Interesting read though.