Depends who you ask. If you ask a music teacher I am sure they will cite chapter and verse for why music is the ultimate learning experience for children, along with many more reasons why they teach it. When I was in school we had to choose an extracurricular activity - band, chorus, etc. If students and their parents did not have to choose an arts class, but instead chose to use that time to double reading or math, it would lead to decreased funding and employment for those teachers.
Ultimately music is a language. Again, there are plenty of arguments about the benefits of learning (a) new language(s).
That said, I don't think teachers can argue in good faith that learning a particular will make you smarter versus learning another. Are multilingual people more intelligent than monolingual? Are people fluent in Ruby smarter than those fluent in PHP?
A better argument would be, learning music could potentially unlock the ability to learn new languages. Debatable, but I'd agree with that.
There is a difference, though, between learning something as a hobby, which is what most people learning music do, and mastering an instrument, which is a lifelong pursuit. The pursuits different, and so are the rewards. Learning something as a hobby will provide benefits, but none that can be associated with gaining more intelligence.
