Of course, once you are a red deck, why do you even need Chained to the Rocks? I guess it is nice if the format ends up having as many green fatties, Boros Reckoners, and Gods as it looks like it might, but it's still a little odd. We don't even have enough creatures to make me want to Purphoros (although it might be that the first one or two cost you so little that you need to play them anyway).

Perhaps the best place for Chained to the Rocks in Boros Aggro is in a balanced build, splitting the colors down the middle but avoiding double cost creatures like Precinct Captain and Ash Zealot.

http://www.starcitygames.com/article/27031_Theros-White-Green.html