If they used all the grass and then lost the engine on rotation they would probably carwheel through the field in front of them (or the building...).
Thankfully, most pilots would not accept this levle of risk - especially as operating beyond the limits of the airframe or operating handbook can prove interesting insurance-wise in the event of an incident. It's not just the losing your licence to the FAA and the claim against you or your estate, it's the prison time that can get really galling...
Best option if you are concerned is to ask the pilot what the minima for the aircraft actually are, and see what the 'spare' runway equates to. Then see if you would be happy to jump a rig with that level of safety margin.
Airline Transport Pilot (ATPL JAA)
