Being faithful to a single style is one of the most powerful aspects of a good garden. Consistency in design - where similar themes are repeated - results in a more soothing space. Some of the big decisions required by the garden designer are formal vs. informal, native vs. exotic, literal vs. metaphorical, man-made vs. natural,... Of course knowing when to shake it up can inject excitement into a design. It is the contrast of one exciting element amidst soothing repetition that makes it work.
Check out posts that show other examples of how gardens use rhythm and consistency.