Why buy ground cinnamon instead of cinnamon sticks? Ground cinnamon is less expensive then cinnamon sticks. I've read that it is also more flavorful. However, cinnamon sticks last a lot longer. Also, ground cinnamon is likely to have mystery ingredients. Will I (a cinnamon nut) be disappointed if I stock up on cinnamon sticks instead of ground cinnamon?
Different applications, sticks are mainly used when you wish to infuse a cinnamon flavour in a dish, where the spices are removed at the end, a biryani is a good example.  Ground cinnamon is used when the spice is to be left in, or be present throughout a substance, like in cakes for instance.

Ground cinnamon will leave a stronger flavour partly because it is left in and possibly due to the flavour extracting easier due to higher surface area and broken structure.

Cinnamon is notoriously hard to grind smoothly, so people tend to buy both for their respective uses.