I always got the vibe that James and Sirius specifically attacked Snape because he was alone. I don't agree that Snape was far greater of a wizard than James, but he did tell Harry at the end of Half-Blood Prince that James always attacked him when he had backup and Snape didn't:
“Coward, did you call me, Potter?” shouted Snape. “Your father would never attack me unless it was four on one, what would you call him, I wonder?”~Harry Potter and the Half Blood Prince - 28. Flight of the Prince
Though he wouldn't admit it, James probably knew that Snape could be a problem if they dueled one-on-one (I don't think it would be an outright curb-stomp either way though).
From a meta perspective though the chapter "Snape's Worst Enemy" wouldn't have been as effective had we seen five more Slytherins rushing in casting curses at James and Sirius. We needed to see Snape as a victim and James and Sirius as the aggressors.
Snape's gang probably did offer him a bit of backup on occasion, and maybe even had their own conflict with the Maurauders. Just because we never see it doesn't mean it never happened.