One problem the brass were concerned about was the ability of the two man crew to hold up during 24/7 operations. The M-109, with a five man crew, has enough people to take care of maintenance, standing guard and, basically, always having one or two people rested and alert. Not so easy when you only have two guys. One solution was to have two or more crews per vehicle, as combat aircraft (and some warships) have done for years. The off-duty crews would be back with the support troops. PIM will have a crew of four, compared to five in the Paladin M-109.
I don't know how rotating 2-man crews helps with security, even though it would help with maintenance and alertness. Plus you have to move crew around a lot more with all the problems that raises (IEDs along the road, additional vehicles to move the crew, fuel and maintenance for that vehicle).
I can't speak to the maintenance issue of a 2-man crew, but we do have a technological solution for security, I think. Couldn't we put a remotely operated CROWS weapon system on the vehicle so that troops in the rear could provide observation and local security while the crew maintains the vehicle or sleeps?
Perhaps we supplement the vehicle remotely operated CROWS with a mast-mounted or lighter-than-air sensor that provides a better view around the vehicle.
There may be many reasons for needing a larger crew. Security seems like a reason that we can work around.
