Lena Groeger begins a stint blogging at Rationally Speaking with this entry, "So, what's science good for?". She briefly discusses the usual rationales for "broader impact" of scientific work, as well as the intrinsic rationale -- science for the sake of knowing. I might call this one the "epicurean" rationale for science.
The meat of her essay is this proposal: We value science because it makes a different kind of person:
What I don't hear much in this Useful/Its Own Sake discussion is a third factor, one that seems to get overlooked in the haste to divide by two. And that's the value to the person — the habits, the values, the character that a person develops by actually doing science. The ability to think critically, exchange ideas, imagine alternatives, ask questions, invent new possible worlds and go on speculative adventures, present reasoned arguments and retain a healthy dose of skepticism and doubt; these are all deeply scientific and deeply formative values. You become a different person with scientific training, and we do science because we value that kind of person. Science is not only bridge-building (usefulness and technology driven) and knowledge-building (adding to the pile of facts we know about reality) it is also people, character, and citizen-building.
I think this comes closest to a reason for science grounded in ethics.
The remainder of Groeger's essay, by comparing the "critical thinking" of the humanities to that of science, I think actually minimizes her point -- and the comment section comes alive with humanities defenders. Frankly, as someone who started in the humanities and turned to science, I do not agree that "critical thinking" means the same thing in these fields.
Oh, as a matter of practice, I think there are many excellent humanists who would make good scientists, and vice versa. There is a common intellectual grounding to the two enterprises. But I think there are logical reasons why the "two cultures" remain separate. They are not at all the same kind of thing.