Quite by accident, I stumbled onto a new-for-me play style: using dragons as companions!

In the middle of a dragon battle, I stumbled onto a bandit-filled shipwreck. Final score---Dragon: 5 Bandits: 0. Good pickings plus a weakened dragon. Can this be done throughout the game or did I just have a lucky encounter?

Are you on PC? Dragons are hostile to almost everyone, it could be just complete chance you stumbled upon one.

If there is one around and you enter its area it will attack anything in sight, not just you. Had one clean out a bandit fort before I could get there then tried to chow down on me, my character however was too tough to chew.

there is no useful dragons as there is no useful giants, they are in the game to be killed

Contrary to what seems to be the consensus, YES, there are random dragon encounters with non-hostile dragons in the VANILLA game.

It is known. http://www.uesp.net/wiki/Skyrim:Random_Encounters#Encounters_With_Differing_Outcomes

Scroll down to Dragon Fly-by.

I could poke around the CK and bring even more technical evidence, but I really don't see the point...

It's true that a few dragons do flyovers. I don't think that was the purpose of the OP though I am certainly not perfect.

What the OP described was exactly that. A "random encounter" with a neutral dragon. They will however attach other hostiles if they come in range of their fly-by area, because of how factions are set-up between actors.

They are set-up to be neutral to the player.

EDIT: they are neutral to other NPCs too (people in towns etc. ) They won't attack unless one of those silly guards manages to get a hit on them.