One of the issues I've seen with the handheld cell phone scanners + thermal paper boarding passes is the boarding passes simply flopping down or needing to be fully extended for the GA to properly scan in mid air - requiring the customer to use two hands to hold the ends of the board pass apart for the agent to complete the scan.

One of the other challenges that most folks have while standing in the boarding line is that they tend to have only one hand free - whether holding food since they're not getting anything on board and/or pulling their rollaboards.

David