That being said, in my opinion, great science is largely about insight, and this does not necessarily depend on maths. For example, consider the contributions Faraday made to our understanding of electromagnetism. Although he never mastered maths, he broke the ground for many that followed him, most notably Maxwell. With Maxwell, you have almost the opposite situation, as he was a highly skilled mathematician: his insights were largely borne of mathematical abstraction.
So I think the best answer to the question is sometimes. Faraday's insights were not (directly) dependent on advanced knowledge of mathematics, but Maxwell's were. We needed the genius of both these men, but mathematics was only required of the second.
What should we do, then? Definitely require hard science majors to learn mathematics, even more so then currently. Encourage mathematicians to think about science problems, and vice-versa. But let us not make the mistake of closing the scientific doors on the so-called 'mathematically illiterate.' For all we know, they carry the solutions to some of our most difficult problems.
Edit: changed 'where' to 'were'