If I could carry my backpack I know it is a great way to get to awkward windows. If you are strong enough then you have got the best system for wfp, residential. But if you are not strong enough then it is impossible. You have to look for an alternative method. But don't knock anyone who is using it just because you can't. Looking at David Salkield's avatar, if that is him and he carries a backpack, well I am sure he makes as much an hour/or does as much work per man as anyone, whatever their system.
Oh and it isn't based just on the system, it's based on the type of building/client that you are working on/for. Accessibility etc.
End of argument