Though It's arguable, it's grey, there are other instances where one must have a 107. Please correct me if I'm off here guys.
If you're flying for "other than recreational" purposes, you must have a 107.
I'm registered with Air Bears, if I get the call to go assist the local authorities in some SAR on a volunteer, good citizen basis, technically I'm supposed to be 107 qual'd. If I go and voluntarily assist a local university in some environmental labs, technically I must be 107 qual'd.
Furthermore, if you fly for recreation or hobby purposes, but in a way that is not in accordance with the FAA's *Interperetation * of Special Rule for Model Aircraft (Public Law 112-95 Section 336), you must have a 107. Example, if I wanna go fly my fixed wing FPV birds out along the beach with goggles on, I technically need to have a 107 (lots more points of argument/discussion here).