It's common usage because in its strictest sense, the word is absolutely useless. Both everything and nothing are perfectly "unique". Every distinct thing is one of a kind, no matter how many similarities it shares with others, but nothing is "without like or comparison", as all things share something in common with others. So, creating a sliding scale of uniqueness is the only thing that gives the word any value. Makes a lot more sense to say "fairly unique" than to say "unique in sense A,B,C, but common in sense X,Y,Z."
