Im not 100% but I think in Philly the idea behind the nail-bomb was to throw suspicion towards a local Roofers Union. In anycase it seems like the idea is to cast dispersions on the origin of the attack; since wiseguys dont use bombs, it must be an outside force...?
As has been said though, every rule has been broken by now. & whose rule? In Chicago car-bombs were a viable threat. Nick Calabrese testified about using car-bombs to kill people for the Outfit.