User blog comment:Willbachbakal/Multiplayer Weapons: Hack Tool/@comment-174.48.166.4-20101219182715/@comment-1330314-20101220161913

The disadvantages are there so that you aren't forced to pick an upgrade. Otherwise, people from higher levels would have an advantage over the newer players, and once you'd have gotten a weapon you wouldn't really want to use it until you got an upgrade for it. Besides, upgrade balancing means you can have a wider variety of abilities: more powerful upgrades could just get bigger nerfs, without making them better or worse than any other choice.