I'd be more concerned about the fire risks of people carrying 3rd party (from deity knows where) batteries around in the bottom of kit bags where they might be pierced by screwdrivers or whatever. Of course the battery could be housed in a 2mm ABS plastic case, but then your phone would contain a total of 4mm ABS that could be better utilised for a bigger battery in the first place.
Whilst the very rare occurrence of a burning phone can cause injury, it's very unlikely to cause death, which an unattended fire (kit bag or paper-filled briefcase left in office) might. And if a phone battery does start to thermally runaway, it's maybe not a bad thing that it's surrounded by glass and metal instead of paper.
Since I have a grasp of statistics, and am a regular road user, the risk of a phone fire is pretty far down the list of things that scare me.