I try to keep park hopping to a minimum. The more times you hop, the more time you spend in transit as opposed to enjoying the park attractions.
But I usually don't care to spend the entire day at a water park, and some folks like to go back to the resort during the day, so being able to return to a different park for the rest of the day is handy.
On Disney transportation, expect to spend 45 minutes turnstile to turnstile when park hopping, if you drive your own or a rented car, less but still a sizeable chunk of time.