While others have noted how invaluable Infiltrator's Arrow is, I would also like to note that, because of the way initiative works, thieves don't really need a second offensive weapon. Other classes swap weapons because they have weapon skill cooldowns. Since thieves don't have weapons cooldowns, they don't really need to swap weapons in combat unless trying to get away or something. In my mind, each thief build uses a single weapon set + shortbow, and that is just fine. It makes our gameplay more focused, and you can get better at using a set much faster if you are only using one. Honestly, this is a great boon, because I wouldn't want to have to constantly decide whether shortbow was worth taking. I like always being able to take it. Of course, shortbow isn't required. If you still want the mobility, rifle #4 is comparable. If you bind a key for look behind, you can use it almost just like Infiltrator's Arrow and still have the much greater damage from rifle, although you do have to give up steal and a spec for Deadeye.