There is a difference between correct and commonly-accepted writing. I write for a legal publication and my wife has written for the AP, and both of our employers emphasize(d) that we write for the masses, ie., for a fifth-grade education level (this was more appalling to me years ago, but it fails to amaze me now). Therefore, I use the commonly-accepted form, "Murphys," in writing for my employer. The great writers in literature would use the correct "Murphies."