This question actually reminded me of an issue with my exterior walls. I have the same problem with the vertical mortar being eroded/missing from several bricks at ground level on the exterior of my house. I live in Alabama, so we don't have much severe weather (unless you count ridiculous humidity), and I have no idea how this could have happened. The rest of the mortar is fine, but only a few joints are completely missing. Is there a way to prevent this from happening? Is this some purposefully built structural feature that I'm just oblivious toward - they seem to be fairly regularly spaced.
When the home inspector came by before we bought the house, he didn't mention anything about them, and I didn't really notice them until we moved in.