Vehicle operations can be a consideration in running slick, and how well you in issued gear FIT in the vehicle.
In 2005, we finally decided the 2nd Brigade SOP would be all pouches would be directly attached to the IBA. Me, I made a deliberate decision with my 1SG's support to run a SOE Hellcat chest rig and leave my IBA slick. I was the 1SG's driver, among other things. I am a 2XL sized human being. The combined thickness of a rear ESAPI, Kevlar, me, more Kevlar, and a front ESAPI basically consumed all available space between the seat back of a M1114 or M1151 and the steering wheel. Ammo pouches just were NOT happening. I threw my chest rig over by the gear shift lever and figured I would grab it in event of bailout. When I traded jobs with the gunner, I did wear my chest rig in the hatch, just because I figured I would pop the Fastex buckles on the shoulder straps if I needed to get out of the rig in event I was hung up. Also had my strap cutter and a couple other knives around in event I was hung up.