Of course, if you build with a lime mortar rather than OPC you don't need expansion joints.
In truth, most modern mortar is ludicrously over-engineed for what it needs to do. A standard 5:1 mix has a compressive strength of around 70Nm/mm, whereas your average housebrick has a compressive strength of around 11Nm/mm. That means that, under stress, the bricks crack first, which is not a good idea (and which is why modern houses are built on massive concrete foundations and why, when you use OPC mortars and renders for repairing old houses, you end up with cracks and damp problems). Ideally the mortar should be softer than the stuff it's binding.
And don't get me started on the wonders of cow-dung - I've got a job coming up next week which will require plenty of the fresh stuff. The client has been warned about the whiff, but says she doesn't mind!