They make TONS of MOLLE accessories, you can basically customize your pack to carry what you want where you want it on them. It's a great idea! I have two MOLLE packs but I hardly use the webbing on them. I plan to though
You can put anywhere from flashlight holders to triple AR15 mag holders on them, canteen carriers, Med Kits, thus saving room on the inside of your pack for other goods.
If you do put a med kit on the outside, just wrap the contents in plastic as best as possible to keep moisture out!
the Molle system is used for customization, if you take a look at the millitary and police forces they have all gone to it because it allows them to buy a standard item (body armor/ back packs/ etc) then each individual can customize where there put ammo, radios, grenades, equipment. For instance you and I can have the same back pack but you are using a light assault rifle as a scout, I have a long rifle and need to be set up for building hides you and I would det up our kit much differently