Number one: It was the quickest craft to ever fly until 1943, so stuff that up your pipe and smoke it with your single seat fighters and inteceptors.
German plane designers must have been fuming, having their purpose built interceptors being beaten by a wooden plane built in a shed.
Number two: It was originally designed as a bomber, so to be able to double up as the complete opposite is a massively handy thing- as i don't think that the FW190 could carry a 4000lb bomb load while it's sister mark carrys 4 20mm cannon and 4 303 rifle calliber guns. EDIT: And another sister mark carrying a 57mm cannon blowing ships out of the sea.
Number three: It was designed so it could be built by people with lincences to build chairs... so the military factories could carry on with making the spit and the lanc..etc.
Number four: How many other planes, especially WWII planes, could do 50 different tasks with only minor changes done to the aircraft?