With as many guns in Destiny, each with unique trees, I'm not surprised the skill trees are the size that they are. Each weapon, I believe, was designed to have an ultimately unique effect and be really cool to use by the time you're done upgrading it, and that is the effect Bungie wants you to have. So no, there wont be variants of one weapon guardians are walking around with, but rather guardians walking around all with different weapons. And in my experiences, evening game sessions are 8-10 hours in length. And note that weapons are upgraded through use.