When you buy a guitar, you're not just buying a functional piece of musical equipment. Even if the guitar is new, there's usually some history of people using a guitar of the same design - and you want to be a part of it. You see the photo of the guy you admire playing it on stage and you say to yourself "I want THAT" … meaning not just the guitar, but the lifestyle and everything that goes along with it. By purchasing the guitar, you're elevating your own life... coming closer to what you want to become. "Mojo" is the magic element that you're buying. It's hyperbole... not a misuse of the term at all.