The real question is not regarding the merit of the work or justifying an application. The real question is in regards to bias on the part of judges against people outside of the photo-community who have established themselves in another field. If the grant is to help the career of a photographer, and the applicant is a doctor who happens to be a serious photographer, will the judges discriminate against the non-photographer no matter how good their work is?
Aaron Copeland was an insurance executive who composed music in his spare time, Cezanne was a banker who painted, and Chekhov and Eliot Porter were qualified physicians. Today we know these people for their art, not because of their other professions. Yet they are among the greats for all time because of their artistic contributions.
Likewise there are many accomplished photographers, who make their livings in other fields, who can pursue long-term projects without the need to worry about commercial concerns or recognition. Are they any less deserving of funding and recognition than someone struggling to make a living from photography alone? Do their other professions not sometimes give their art a depth, maturity, stability, and perspective that photography alone would not? Being an amateur is something to celebrate!