I've recently started turning mine off as it seemed to be "leaking" data even though my Thai SIM was selected as the data SIM. A phone call to O2 got me a credit for the leakage but in order to stop it happening again I leave it off, and every day or two I disable mobile data, turn the SIM on and it lets me know any calls I've missed and downloads any SMSs. Once done I turn the SIM off again, enable data and I'm good again for a day or two.