I just started building a support thief and initial testing in random PvP matches and PvE dungeons with my guild have been very fun. I run 0/0/30/10/30 traits and i’ve found that as long as you are smart with your positioning and read the boons on your opponents quickly you can make a huge difference in a team of three or more.
As others have stated, shortbow is the best blast finisher there is, and the importance of venom/stealing here lets you save your initiative to spam it. You deal very little damage on your own but in a large coordinated group, spreading basalisk venom to everyone is incredibly effective. Likewise, skale venom stacks a great deal of vulnerability.
Stealing is the most important piece of the build, and in my opinion the trickiest to use. You need to both target an enemy with useful boons to steal and activate it near allies for the Fury, Might, Swiftness and Vigor boons. At the same time you are ALSO searching for enemies that can be interrupted effectively with a daze. Pulling off the perfect steal is rare at first, but practice does improve your ability.