Basically every single piece of music ever that has been vaguely listenable, has been based on SOME kind of musical theory. It might not be always the same musical theory but there's always something in it that fits to some kind of order, not just some random rubbish - whether that was intended or not.
That's it.
@Venus: Please don't overuse capitals, it makes it seem we are arguing about this. But anyway, do you really know what is music theory? It's a study of what patterns composers use on their music and what feelings they represent. For instance: the most used combinations are Minor and Major, which
usually represent, respectively, sad and happy feelings. For instance, the song "Happy Birthday to you" uses melody and harmony in Major tones. But if you transfer it to Minor, it becomes something more similar to funeral music (yes, it sounds really creepy). And if you manage to combine major and minor correctly in the same music you can express various different emotions. Similarly, music theory also studies rhythms of music. You can have 4/4, 3/4, 6/8, etc. 4/4 is by far the most used rhythm, because it's easy to manipulate (all the intervals of time in the music are multiples of 2, so that you can balance it well). 3/4 is the second most used rhythm, it's
usually used in waltz and ballet, because it fits the rhythm of this sort of dance. However, 5/4, 7/4 and 9/8 sound unusual to our ears, and it takes some time until we get used to the rhythm of the song. It's not considered a bad pattern, it's only unusual. These rhythms are also much more difficult to compose in, because you have to use notes in intervals that sum 5, 7 and 9. But I'm sure that I've learned a beautiful Beethoven sonata in 5/4. Though I've only seen one 7/8 track so far, it was a horror theme. I've composed a 9/8 music once, because it wouldn't fit the 4/4 key, but it really isn't common. It's also possible to compose part of a song in one rhythm and the other part in another, but you would have to add a pause so that it doesn't become a too sudden change and sound difficult for the hearer to assimilate.