It is neither fair nor smart to say "Let's grab money from a successful department and use it to prop up an unsuccessful one."
It would not be smart if your only goal is to run the University as a business, where you cut unsuccessful revenue centers and fund/build/grow the more successful ones to focus on profitability of the corporation. From that respect, economy of scale works the same way as it does at Coca-cola or Wal-Mart. Cut the under-performers. It is cheaper, easier and more profitable to pump out 10 million of the same widgets than it is to pump out very small batches of all-different widgets.
If you are a believer in the concept of academic freedom and in the power of diversity of knowledge and thought (idealistic, I know) then it is vital that more successful departments fund less successful ones. I, for one, want there to exist people who study Latin, despite there being a limited usefulness for it as a career. I want people who study ancient Macedonian philosophy, basket weaving, Sanskrit and all the other fields that most people might deride as training for a career at McDonalds. I want there to be someone who knows everything there is to know about the inner politics of ancient Sumeria. The sum-total of human knowledge is vast and it is important that it be preserved but also expanded with the rigor of academic scrutiny.
I want this done, because the concept of Academia demands it. If we churn out millions of kids at a time all with the same thoughts and ideas gleaned from mass-market jobs training programs, we will lose the intellectual diversity that is needed to preserve academic and scientific expansion. There may be nothing that someone studying ancient Indian tapestries can ever tell a nuclear engineer that will advance his work, but both types of people are necessary to increase the useful progress of art and science.
I understand that the bills need to be paid in order to keep the lights on, and also that there are fields that have much more use in the real world as careers. There are certain fields that have more utility in advancing cutting-edge science and, rightly, should receive more attention for their greater potential to advance the human race. However, we shouldn't neglect more arcane knowledge entirely because of this. The more popular fields need to subsidize the less popular ones, less we risk whole branches of study dying off. This is not the most efficient method of creating profit for the university, but that shouldn't be what universities are all about. They should be about increasing the sum-total of human knowledge in all branches.