MJ was not a magical fix me up, but she saved Peter from possibly becoming Frank Castle and killing Osborn over Gwen. Keep in mind Castle first appeared only 7 issues later in ASM 129, so the idea of such a revenge minded vigilante was on the mind of Gerry Conway. I would also note that there are only 4 characters: Peter, Matt Murdock, Miles and Clint Barton who have hidden identities ( and only Peter and Matt are really necessary for the character), having her helps to keep Peter relevant instead of just becoming 24/7 Spider-Man.