The point of terrorism is to make folks afraid of YOU. If you don’t claim credit, you aren’t a terrorist. Of course not all who claim credit are guilty.
Not always true. The point is to make people afraid, but not always of the source of the terrorist action. The terrorists may want to implicate others. Shiites may want to implicate their Sunni enemies for example. Let the Infidels thin out the "other" side.
Only for the unsophisticated.
A more skilled terrorist wants to damage his enemy. The anthrax attacks caused great economic harm to the US, and changed the way we operated in many ways. Even now, congressional offices want email or faxes instead of old-fashioned letters.