Schoenberg wrote a few books on composition. Here's a quote from one of them that I found interesting(and agree with more as time goes on):
"A composer does not, of course, add bit by bit, as a child does in building with wooden blocks. He conceives an entire composition as a spontaneous vision. Then proceeds, like Michelangelo who chiseled his Moses out of marble without sketches, complete in every detail, thus directly forming his material.
No beginner is capable of envisaging a composition in its entirety; hence he must proceed gradually, from the simpler to the more complex. Simplified practice forms, which do not always correspond to art forms, help a student to acquire the sense of form and a knowledge of the essentials of construction. It will be useful to start by building musical blocks and connecting them intelligently.
These musical blocks (phrases, motives, etc.) will provide the material for building larger units of various kinds, according to the requirements of the structure. Thus the demands of logic, coherence and comprehensibility can be fulfilled, in the relation to the need for contrast, variety and fluency of presentation."
I also strongly agree with a couple things rainwarrior mentioned, about 'stealing ideas' and the importance of keyboard skills when it comes to voicing your chords.
"Stealing", not alluding to plagiarizing but simply being inspired by anything and not being ashamed of utilizing it, is essentially using ideas knowingly from a concept or situation. Relying on your own tastes and enjoyment of certain types of music is one factor, but another important one is understanding how to 'properly' analyze melodic, harmonic and rhythmic components and structures from classically established methods. It is an enormous asset I think.
For example, if you want to study song introductions within certain styles of music, how would you go about doing it? What what would you actually look for and focus on? Surely, using your own established musical senses would take you somewhere, but without a firm understanding of basic and classically established concepts such as harmonic texture(homophonic, homorhythmic, polyphonic, etc), phrases, motives, harmonic rhythm and such things, you'd be missing out on a lot of information and ideas that the music basically hands over to you without much resistance.
And regarding keyboard skills, more 'professionally' established composers than you might imagine learn how to play the piano despite it not being their main instrument. Guitarists, drummers, brass, woodwind and string guys all seem to learn,at least basic to intermediate, keyboard skills at some point in an ongoing professional career. And I keep pointing out "professionalism" here as way of saying, "a strong, demonstrable desire for continuous progress in their craft."... hey zan, stop looking at me like that...
But it's very valuable for arranging your composition, which is something else that's very important: the composition's arrangement. It's one thing to establish a chord progression and melody, the core composition, but voicing chords, building textures and assigning them to different timbres is a much more complex and involved skill. The keyboard is almost essential in developing more complex arrangements and still maintaining communicability between the features of the piece.
But yea, anyway I think classical music is so extremely important and valuable. I'd argue that any progressive musical movement in the past hundred years involved major players with classical backgrounds. Being able to read it, and understand the elements and arrangements involved, without much effort would feel almost like cheating, lol. It'd be just like having a little gremlin on your shoulder that constantly feeds you ideas.
Here's that Schoenberg book if anyone's interested: http://docentes2.uacj.mx/museodigital/teoria/Teoria_musica_2/Schoenberg%20Arnold_Fundamentals%20of%20Musical%20Composition.pdf
. Personally, I haven't really read too much of it, as I didn't feel like it could really help me too much, but I realize more lately that this was only because I really couldn't understand a lot of the verbage and dismissed the whole thing as, "lol, this is way too analytical, Schoenberg you dingus." But I'm finding it's actually quite practical and useful once you make it past the verbage hump(it basically just breaks down how to analyze shit from a classical standpoint and presents ideas on using what you find from this to adopt into your own musical language).