Why you ask? I complete nightfall and heroic strikes to earn strange coins I also do some heavy grinding to earn strange coins. So when I purchase an exotic from xur I earned it. When I just land on a patrol on earth or wherever it may be and kill a random lvl 5 and get a legendary drop that turns out to be an exotic I call that luck. So let's get this straight buying from xur is earning. Getting it else where is luck. The one true way to earn is by completing exotic weapon bounties or the raid.