The electronic safety aspect is (supposedly) covered by the onboard station which will facilitate reduced power transmission by the phone itself. Without the picocell then the cell phone will hunt at maximum power. Is that what you heard Merc pilot? Pretty annoying I'll bet. On approach as well? Ouch.
The physical safety aspect (as described by outoftheblue) involves not waking exhausted travellers up with "I'm on the 'plane..." etc.
