The one disadvantage of backpack weapons is that the enemy can now see the loot that you're secretly stashing away, and this will change battlefield tactics a bit.

e.g. in Halo 2 xbox, if you had the energy sword, equipping it in full view would prompt other players to move far away. However, a skilled player would holster the sword until the opportune moment (i.e. within lunging distance) to trick the enemy. Looks like we can't do this anymore.

Of course, this will go into Halo 3, and it looks cool/realistic, so I like it.