For instance gutters that are filled with debris can lead to several damaging conditions. In our area they lead to ice dams in the winter and in the wet spring they become so heavy they pull the fascia boards off the house leading to rot and eventual structural damage costing thousands.
Think too about siding. Many homes here have vinyl siding. Left uncleaned it becomes a breeding ground for mold and other allergens that can cause illness. Also vinyl siding will break down over time in sunlight. Regular cleaning can reduce this and extend the life of your siding.
Last but not least your windows. Improperly maintained windows eventually become horribly stained which leaves you three choices - 1) heavy chemical restoration, 2) mechanical grinding restoration, or 3) replacement. Again costing many times more that the cost of proper maintenance cleanings.
So it's up to you - how much is peace of mind and a properly maintained structure worth?
For more educational topics go to A New View Exterior Cleaning