According to the article some time back in the IEEE Spectrum magazine, one problem is that although the cabling and so on is adequate for protection of aircraft systems when new, it doesn't always maintain that integrity.
Because of the large number of different designs of 'phone, it's also hard to tell whether or not the spurious emissions will cause trouble - even though they are low eneough to meet the specifciations, they could still cause trouble if they were on certain frequencies.
You don't need to think about jammers causing trouble. How about airfield radars - like that big one at LHR? That's more than 2 watts - nearer 2 gigawatts effective radiated power.