As someone on the receiving end of these sorts of temporary supplies, I'd sure like the distro to have a main switch. That way when the supply loses it's neutral I can try and contain the scale of the disaster...
Its also a good idea for the main switch to be a main breaker, as then the distro is protected from the lack of upstream overcurrent protection, which is common in these circumstances. You arrive at the site, and the house or site electrician tails you in to a 400A supply, despite the fact the cable from the tails to the distro and the distro itself is only rated at 200A. You smile sweetly, thank the electrician nicely for giving you power, and just get on with it.