At the end of the day its all down to cost , not long ago we had tonka/jaguar/harrier all doing different jobs with some overlap and as mentioned before once we'd lost the vulcan the tonka took on that role. Also look at cost history when you look at running costs in the tonka/harrier debate. Its only in the last few years that things changed to get a more cost effective maint system going that got the harrier costs down and availability up.