The advantage of having a specialist is that, simply, she's a specialist. General practitioners don’t perform brain surgery; neurosurgeons do, because they're specialists. The same goes for education. The educational and performance background of music specialists render them the best prepared music educators. Math teachers don’t teach French, because they're specialists in math. The same goes for music.
Students learn by doing. When a math teacher teaches a new concept, she has her students practice it over and over to ensure they've learned it. The same goes for music; students learn by doing. Teachers who aren't music specialists are ill-prepared to place the requisite focus on performance. In some instances, generalists are content to play CDs as a way to teach music, which is akin to showing children a book and claiming you're teaching them to read.
Music specialists are uniquely trained to not only teach music, but teach music using principles learned in other subjects. For instance, time signatures are presented similar to the fractions students see in math class. Quarter notes are ¼ of a whole note—a concept that also applies in math. Musical passages contain repeated phrases and themes; so do stories. Just as the Big Bad Wolf repeats the theme of huffing and puffing, so, too, does Beethoven repeat one of the most popular musical phrases ever in his “Fifth Symphony.” Musical specialists are prepared to find and explain these connections for a broader integration of music across the educational spectrum.
Finally, and quite possibly, most importantly, studies repeatedly show that musicians have higher IQs than non-musicians. They consistently perform better in speech and language, demonstrate improved memory and language skills, and record higher grade point averages than non-musicians. By laying a strong music foundation early, schools aren't only providing music education, they're giving their students the tools to perform better across the curriculum...and music specialists are best prepared to lay the strongest foundation.