I just remember our Protest alliance getting into a lot of trouble from the lovely "Robocop", whom we lovingly referred to with the second syllable replaced with a well known abbreviated term used for a male chicken.
i'd prefer no arson, but yes, there are definitely decent possibilities by having 1 arson in your line up.
basically you'd be using that person to smash and grab, but they would be 0 muscle in a fight. poms/gurus block better than arsons. pb's and attack dogs better offensive/defensive options if you want hooligans for anti harrier.
so it boils down to what you want of course, and definitely that you would HAVE to put that route into the right hands to make the most of the choice.
i'd prefer pb's in the hands of a fast starter or attack dogs in the hands of an active solid player.
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.