The periodic replacement of mortar insures the future health of masonry. This process generally needs to be completed about once every 100 years. If mortar is cracked or powdering, water is likely to penetrate the wall. In lime mortar buildings, when the mortar has turned to dust it has come to the end of its useful life and needs to be replaced. If your mortar is worn out of the joints, substantially cracked, or if you see damage to the masonry, replacement of the mortar is required.
Hairline cracks in mortar suggest the use of Portland cement. Small cracks cause more problems than big cracks. Capillary action allows water to be drawn into the wall, but then the water is trapped behind the mortar where it will tend to migrate towards interior plaster while deteriorating the remaining lime mortar.