Many folks like to shear their plants into balls and cubes. I really don't. I like my plants to have a nice appearance, but with a bit more loose, natural look.
Beyond appearances, the main problem with shearing is that it ignores the underlying structural issues within a plant.... and, it's whats on the inside that really matters. Below we've highlighted some common structural problems with trees.
This oak has co-dominant leaders. The "fork" in the main stem will become a weak point in the future so should be removed during it's youth. Remember to only prune oaks in winter to reduce the spread of disease.
This Linden has many structural issues including the crossing branches at the upper left corner of the photo. As the tree grows these branches will expand in size and literally strangle each other. In less severe cases where branches are simply touching, the repeated rubbing from swaying in the breeze creates wounds that weaken the wood and can allow disease to enter.
This Freeman Maple has tight branch angles that are prone to splitting during heavy winds. Due to it's location between two homes this is concerning. Considering the fast growth of these trees we recommended removal while the tree is immature and replacement with a more sturdy, long-term tree.
This Linden has tight branch angles on its multiple stems. As each stem grows they will compete for space and ultimately "push each other apart" causing the tree to fail. Part of the issue with tight branch angles is the "included bark" that stays between the two branches as they grow together. This bark creates a weak point - prone to splitting.
Similar structural issues happen with smaller shrubs too. Simply shearing only addresses the outside shape - not the structural problems.
Another reason not to shear is that shearing enhances density on the plants perimeter so much that there is little air flow through the plant. Loosening up the exterior and letting the air flow through helps reduce fungal problems.
For more info on pruning appropriately see http://hort.uwex.edu/articles/tag/pruning/