Masonry repairs to the chimney are very common in most older homes. Cracked chimney caps are probably the most typical. A mortar wash as shown here will crack allowing water to get into the chimney itself. Cycles of freezing and thawing can ultimately cause damage to both the brick and the fireclay liner.
As with most maintenance items, the longer this goes unrepaired, the more expensive repairs become. In severe cases of neglect a chimney may have to be taken down and rebuilt. It is always less expensive to keep up on maintenance then let repairs go until damage reaches a critical level.