I think you overstate the case -- for the present.
Thought experiment: Imagine you could magically transport several humans to Mars along with all the shelter and supplies they needed. Naturally, you could also use your magic transporter to transport a robotic vehicle. Which would be more valuable?
At present, the humans would be a better choice due to their greater behavioral flexibility and autonomy. But over the next fifty years or so we can expect the gap in flexibility between humans and machines to narrow. In 2064 we might prefer to send robots through our transporter, simply because of the logistics of maintaining a pressurized environment. We're already finding applications on Earth where we prefer to fly drones rather than manned aircraft, and not necessarily because of safety.
Yet at present we still find it more convenient to do some things in low Earth orbit with people rather than robots. The cost and complexity of maintaining human life 200 miles away is plenty high, but it's still worth doing. In a hundred years, maybe not.
Here's what I think the lesson of the thought experiment is: the choice between a future manned expedition and a robotic expedition, reduced to purely practical concerns (i.e. excluding things like glory and adventure) will come down to the rate of marginal advances in robotics vs. marginal advances in space transport technology.
At present the state of space transport technology favors sending robots to Mars exclusively. But how do we advance space transport technology to make the manned trip desirable? Well, there's some engineering research needed of course, but the best way to gain practical experience in the short term is to send more robots. If we do *no* robotic space exploration, the advantage will shift even more towards robotic exploration, because space technology will stagnate while robotics continues to advance. If we want to see manned exploration of Mars in our lifetime (for those non-practical reasons above), our best chance starts with an intensive program of high-risk robotic missions.