Tough question. Honestly, I don't believe that there are any disadvantages to having art in a curriculum. I think it is an integral part of teaching the whole child.
Unfortunately, not everybody feels the same way, which is shown by how many schools have cut art programs from their curriculum. I can lay out a few of those arguments for you.
One suggested disadvantage is that running a robust art program is expensive. There is some merit to this argument. Art supplies cost more money that math supplies. Many people feel that money should be spent on "reading, writing, and arithmetic." Those are fairly cheap subjects to budget for.
Additionally, schools right now are making big pushes toward STEM. Science, technology, engineering, and math. Given the choice between paying for art or paying for STEM, schools are choosing STEM, because that is the big push in education right now.
Another possible disadvantage to running an art program is that art classes are generally not required courses. That means class sizes are likely smaller. That's great for an advantageous student/teacher ratio. Unfortunately, budget wise, it looks like a teacher is being paid equal amounts of money to teach fewer students.