He is essential.
Something I noticed about M'aiq, he seems to lead you to unmarked quests.
Once I found him near Riverwood, he was attacked by bandits and I sprang to the resque. Afterward I looked to see where he had run off to and he was standing by the body of a dead woman. She had a note on her about a traders life being difficult these days with all the bandits about.
Another time I found him standing beside an Orc.
The Orc asked me to give him an honourable death, same as the Orc Umbra in Morrowind.
He was just a standard Orc though, not very tough and with no special loot. Just a guy who didnt want to become old and grey.
He doesn't lead you to unmarked quests, all field events have set spawn points at which certain (all?) events can occur and the game loads a random event or events (or none) from that list at said locations as you cross a certain distance threshold. M'iaq just happened to share event locations with those events and they both loaded at once.
I noticed this when I saved in the field and crossed a hill to find a daedra hunter, then reloaded and crossed the same hill to find 2 necromancers, then reloaded and crossed the same hill to find a dead treasure hunter.
Field events have set spawn coordinates. Guidemakers start your maps!