I'm sure you wouldn't object to national parks or wilderness areas, it's just that in the case of Mars there is no (known) life. How much each of us values the presence of life will differ, but in my view it's worth preserving Mars.
I'm surprised that a researcher wouldn't be enthusiastic about being able to do science directly on the planet's surface. Sure, an extended human presence there will require a lot of supporting infrastructure, and sure, that will all have a direct effect on whatever might already be there -- or might not be, and in either case, changing that environment might not be desirable.
But even at the current faster speed of remote exploration of the planet, you can expect, what, like 3 more robotic missions to Mars before you retire? And each one will be an iterative improvement over the last.
There must be so many questions you have about the planet that just won't be answered in your lifetime unless we go there and have a look around. What's your position on that?
I am not opposed to scientific exploration at all, (I work with several people who run Ops on MSL and Percy), but putting people on the surface requires much more infrastructure and expense. A sample return mission is already extremely complex and costly, and would be orders of magnitude more so if the mission also had to support human life.
Further, terraforming Mars so that people could go explore it seems completely backwards, since you would be altering the conditions you would be studying.
To your question:
>There must be so many questions you have about the planet that just won't be answered in your lifetime unless we go there and have a look around.
I'm not even sure what those would be. Almost any question we could want answered could be solved with a probe or a rover. Obviously there are questions we can't answer right now, but those are largely because we send rovers that are specialized in some areas, and totally ignorant of others. To answer those questions, it would be simpler, cheaper, and better for Mars to send more robots than to send people in my view.
There are millions of Mars-like planets out there in the galaxy. I'd be more than happy to preserve, say, 10% of them. But there's only one that's in the right place to be the next step for humanity.
Turning choice, large asteroids into liveable habitats seems an infinitely more productive task.