An interesting question. I would suggest that the role of a designer necessarily requires a purpose or motive for the design. In the case of life that would require a purpose not just for humans but for all living organisms from the most basic to the most complex.
The purpose of the design will dictate the nature of the designed.
A world created by a benevolent God would be very different from one designed by a malevolent one. A world designed to achieve a fixed purpose would be very different to one designed as an experiment to see what happens if a number of random factors are thrown together.
I leave it to the individual to decide which our world most seems to resemble.
My point is that the purpose of the design would logically define the role of the designer and ultimately the nature of the design itself. If we ask the question of design we necessarily ask of the purpose of the design. Does the existence and design of life as we know it achieve any discernable purpose that any sort of designer could have hoped to achieve? Why are we here.........?
Interesting. However I think there is a flaw in your argument. You say -
Now the Earth, I think everyone will conceed, is quite stunningly beautiful; and it is noteworthy that the principal exceptions are those parts which have been modified by humanity for practical purposes
But if we are part of the design then the design itself is inherently setup to become increasingly less beautiful and therefore less ornamental over time (i.e. as we continue to screw up the planet for our own practical ends)
The obvious conclusion is that the ornamental design you describe would be infinitely better designed if humans had been completely left out of the equation............
Are we the equivelent of termites eating away at the beautifully painted canvas? Or, to continue in the spirit of your argument, are we in fact the ornament? The equivelent of exotic fish in huge fish tank.