No chromatism, no augumented or diminished intervals, no sevenths.
It was not until 150-200 years later all intervals within the octave were accepted. Why and when were "moving octaves and fifts in parallel "forbidden"? It was not until around 1800 resolving the third (e in a C) to the fifth (D in G7) became widely used. (some composers had used this technique before that, Bach for example).
And so on...If you want to compose film scores or game music and you are asked to compose (or arrange) something specific (and I think that is not so umcommon) like "we want music form this era or from that culture" I think knowing music theory is a must. Another important thing is of course instrumentation. When did the piano of today become wide spread? Maybe you want to compose in the style of a cerain era but someone thinks that the great oboe sample you used sounds strange since the oboe did'nt existed back then?
Sometimes it's very good to know "the rules". And in this context - obey them 100%.
I love music theory I am all for it but training yours ears and learning how to listen is even more important:love: