I personally think that "double-click is used" simply because 
"double-click is used".  In other words, it's a circularity.  People 
continue to implement it (in other systems) because people continue to 
implement it (in other systems).

Taken at face value, though, and without the baggage of legacy habits, I 
personally think it is a bad metaphore.

In those UI systems that provide the ability to eliminate it, I've 
turned it off as a UI option, and after awhile, I find myself 
*preferring* to leave it off, even though overall I've had more 
experience with it on.  Thus I don't believe it's just a matter of what 
you are used to.

Nevin