On top of the usual problem of unmotivated actions, it eases the path of your hero no end. It is never wise to ease the hero's path (until the denouement, of course). And it makes it clear that the universe is not a place utterly hostile to the hero, which is even less wise.
I've never really had a problem with such characters in my stories. Because while I have had people showing up out of the blue to help the heroes of them, they always come across as frightening and unsettling to the hero: people of mysterious motives, how can he possibly trust them? Even when he has no choice but to accept their aid.
They don't show up very often. Usually, the helpful characters not only have axes of their own to grind, they make it clear up front. Which can be very unpleasant for the hero but does establish their motives.