Pretending "aesthetics" don't matter and our hardware isn't very significantly designed to respond to emotion is a huge flaw in "purely logical" reasoning. For goodness sake didn't we already cover this in Star Trek TNG?
So let's just skip the trip to Mars and spend the money to build high-tech pyramids in the Nevada desert and sacrifice virgins.
It's still far off in the future, but we have to take one step at a time. One of those steps will be sending people on a mission there with a return ticket, eventually establish a continuously manned research base etc. etc.
So there still may be some utility in the long run.
Of course there might be other approaches to bootstrap space colonization, such as the idea of mining asteroids in orbit and maybe sending people up there in the process.
The robots sent to mars are not useful for any reasonable economic reason. Even insofar as you count speculative basic research as economically useful, there are vastly better ways to spend that money on speculative basic research on Earth.
So why go to Mars? Because either (1) you find the basic research being done about Martian geology particularly compelling or (2) you want to see it explored by humans for its own sake. If you really actually believe in (1), then by all means; send more robots. But I'll bet 100:1 that you personally have no appreciation for the basic science questions, which are mostly very technical and boring. (Hint: grand-seeming questions about life are promoted for publicity and funding reasons, but these are not the ones that actually drive the science, and we are not learning more about the origin of life from going to Mars.)
Rather, I think you'll have to admit we want to go to Mars because it is awesome. You may think sending metal under our control is just as awesome as sending flesh and blood humans, but ultimately this is an aesthetic judgement which is not grounded in rationality. Personally, I would like to see humans walk there, and I'm happy to pay my citizen-share of $100/year over the next decade to make it happen.