I think there is canon support for the administration of actual, physical protection by one's fellow Death Eaters or the groups on their way to becoming Death Eaters post-Hogwarts.
‘As [Tom Riddle] moved up the school, he gathered about him a group of dedicated friends; I call them that, for want of a better term, although as I have already indicated, Riddle undoubtedly felt no affection for any of them. This group had a kind of dark glamour within the castle. They were a motley collection; a mixture of the weak seeking protection, the ambitious seeking some shared glory, and the thuggish, gravitating towards a leader who could show them more refined forms of cruelty. In other words, they were the forerunners of the Death Eaters, and indeed some of them became the first Death Eaters after leaving Hogwarts.’
Half-Blood Prince - pages 338-339 - Bloomsbury - chapter 17, A Sluggish Memory
This is not directly written about Snape, but Dumbledore acknowledges that affiliating one's self with the Death Eaters seemed to carry with it some insular protection. No, we didn't see anyone come to Snape's aid (except for Lily) when James was bullying him in Snape's Worst Memory in Order of the Phoenix; however, Snape cast Sectumsempra¹ on James in retaliation and did manage to cut James's face open. He managed to get a shot in. Even the tightest of groups cannot be together 24-7, especially with each member having a different schedule, etc. There would have been times that Snape was alone and vulnerable to being targeted by the Marauders. Had his fellow Death Eaters been there, though, I believe they would have stepped up to the challenge, if for no reason other than they derived pleasure from cruelty.
Three of the Death Eaters swerved and avoided it, but the fourth was not so lucky: he vanished from view and then dropped like a boulder from behind it, his broomstick broken into pieces. One of his fellows slowed up to save him, but they and the airborne wall
were swallowed by darkness as Hagrid leaned low over the handlebars and sped up.
and
This time a great net burst from the bike’s exhaust, but the Death Eaters were ready for it. Not only did they swerve to avoid it, but the companion who had slowed to save their unconscious friend had caught up: he bloomed suddenly out of the darkness and now three of them were pursuing the motorbike, all shooting curses after it.
Deathly Hallows - page 53 - Bloomsbury - chapter 4, The Seven Potters
Canon examples of Death Eaters assisting each other when one was in physical danger.
I'm not so sure that James Potter was as big an impetus to Snape joining the Death Eaters as he's made out to be. Snape was pretty obsessed with Dark Magic; Sirius described Snape as arriving at Hogwarts at eleven knowing more curses than most seventh years. That's some serious interest in and dedication to the Dark Arts. Canon suggests there were multiple reasons why Snape chose the Death Eaters.
¹To clarify, Snape cast a spell that cut James's face open. It is not implicitly stated that it's Sectumsempra; however, it is strongly implied. In Deathly Hallows (chapter 5, The Fallen Warrior), Lupin notes to Harry that Sectumsempra was always a "specialty" of Snape's.
