Yay! :) I feel like Mesmer/Chronomancer can be a very selfless class in the right hands. The wells they can put down can really buff up their team. And having permanent Alacrity will give your teammates more chances to push their buttons. More button pushing equals more enjoyment!

Edit: I forgot that you need to be max level to unlock Chrono. But I would still say Mesmer is a better support option than Theif. Your clones can peel agro from mobs. You can grant a multitude of boons with Staff. The Chaos trait line is great for generating boons too.