I am thinking the reasoning behind the mulch and the gravel is that mulch is more work to keep up with than gravel over time. Putting mulch over the entire area would create a larger amount of upkeep later. Putting the entire area in gravel would not look pleasing-too much. I think they were trying to create visual interest by using different types-texture and colors ect, since you perhaps wanted minimal plantings due to the dogs. Not saying I would have did it this way, but just saying that may be what their ideas were. I would wait a couple of years for the plants to fill in and then look at it then to see how you want to make changes. Another thing to consider is changes in your future. Do you plan on having dogs after these? Maybe smaller dogs? Do you plan on having kids? Maybe you will retire in a few years and need an area to entertain friends more often? ect. If you plan on keeping large dogs long term over the years, I suggest trees, shrubs, maybe create a shade garden with decorative fencing around it so the dogs don't have access-give them their own fenced space.
