[cdubzzz]

Per my comment, this would only be true if you feel that absolutely everything is art. That sounds kind of nice, actually (:

And how would you decide if a definition (of anything) qualifies as "useful"?

[pegasus]

Concepts are useful because they help us discriminate - between that which falls under the umbrella of the concept and which doesn't. An attribute that applies to every possible thing equally is redundant, and can be ignored without loss of information.