haha yea that thing is funny
im gonna send it to my friends

Well, while very funny, that article about the Aircraft Carrier and the Lighthouse is actually a joke / urban legend:

http://www.snopes.com/military/lighthouse.asp

And as for the ones advocating making the English language more efficient, check out Melvil Dewey (the same guy that gave us the Dewey Decimal System to categorize library books):

http://en.wikipedia.org/wiki/Melvil_Dewey

To prove he actually means business, he almost changed his own name to "Melvil Dui" but settled for "Melvil Dewey" leaving his last name untouched...

Now back on topic...  Names and labels are just ways for people to simplify how they communicate.  Since Parkour, Freerunning and Art du Deplacement are all sister disciplines with the same root, I believe they can be used interchangeably to the layman.  To practitioners who want to make the distinction, however, a different label should probably be used when you need to emphasize a different aspect or connotation attached to the label.  However, please keep in mind that language is a fluid thing that evolves, so please refrain from being a "Parkour Nazi" and chew people out if they don't use the term you expect them to.