Stargazer: I'm fan of having one thief in a setup. It offers a lot of possibilities in terms of tactics. It gives you the option of forcing the enemy to fight on your terms, and it gives you another layer to fight off dogs which are so popular right now.
However, for anything to work you need that player in the hands of a good and non-egoistic player. He needs to be high ranked to have an impact, but he also needs to understand that he's sending few to nil geos on your ally attacks, lest he grows too big on behalf of the other players.
Finally, for this to work at all you need to have a lot of competent players filling the other key roles. If you hinge your best player on an arson, and lack the quality in your other players to make an effect as their route, you're going to lose. Because as Garrett said: you'll be lacking the firepower from that one players, so you better make sure that you have enough key players in key routes to hold their own while you exploit the route.
It sort of falls in the same box as extremists (altough there is a distinct difference between the two), in the sense that I would gladly draft an extremist player due to the tactical possibilities you gain from it, but I never seem to have room for it. Maybe if the we had 25 players in each ally, but.. I'm digressing.