I find carrying a bag that is shaped like ?[] is an easier carry than one shaped like ?[__] (Tall and thin rather than short and fat--question marks are pack straps)
There are three advantages to the pouches for me at least:1) You can slim the pack down when you don't need to carry loads of gear;2) You can can pick and choose pouches to suit what you want to carry, which in general means that it'll perform better for a specific purpose;3) It's easier to find and grab stuff of you don't have to root around in a single large pocket where all your stuff resides.I'm guessing that those three reasons are part of the overall reason that NATO armies have standardised their webbing into the MOLLE system I should add a reason 4 - it's fun too