I keep song lyrics and set lists on my phone so I sometimes could be seen with me Blackberry in me mitt.... So there are a number of more acceptable reasons for brandishing a mobile phone in a session or gig.

Whilst I experience mild irritation at people being glued to their phone in a concert or session (obviously texting or playing at summat) as long as the phone is on silent then there is little real disturbance caused. As others have suggested, I assume that they are either there under sufferance or have a short attention span.