Lingua francas exist because they allow people with different native languages to communicate, with an emphasis on flexibility to the detriment of rigor. Were a lingua franca's spelling made strictly phonetic, it could result in different spellings of the same words depending on the native language of the speaker, which would hurt communication.
English is currently the global lingua franca, and mostly as a quirk of history has much less phonetic spelling than comparable languages. I would argue that the relative divorce between English spelling and pronunciation helps it be a lingua franca, and that English's role as a lingua franca retards efforts to rationalize English spelling.
Have lingua francas historically resisted phonetic rationalizations, and have they lost lingua franca status when such efforts have altered the spelling of words?