In general, you try and prevent rain running down the wall in most
conditions. That's what guttering is for.
Driving rain is not uncommon.
And if your theory is correct, you'd never get damp in an upstairs room.
You get less upstairs
Oh I can assure you those 'damp meters' can show the presence of water in
apparently perfect 'old' plaster. Certainly did in my case. But it's still
there some 30 years later, looking fine.
Then you don't have a damp problem
You mean the original mortar the bricks were laid on? If a wall relied on
this being a perfect seal, they'd all leak. Have you never dismantled one?

The pointing is what matters.
So we're relying on 1/4" of sand and cement, perched precariously on (and
invariably not bonded to) 100 year old bricks are we? Try that on a SW
facing wall.