#1
I've been coming up with some material and I was wondering if you guys think it'd be worth learning drums if I'd only use that ability for song writing really, or should I just use a drum machine to come up with the tracks until I can form/join a band and show it to the drummer to play.
#2
Worth it imo. It'll give you a much better perspective musically, and you'll also get a good idea of what you can actually do on the drums.
I started playing recently, and I am having a much easier time writing drum parts since I started to get really into it.
Oh yeah...I almost forgot...it's a lot of fun playing as well!
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#3
There's no need to actually learn drums just for songwriting, so long as you have enough of an idea about how to utilise them within the song you're working on.

I use a drum machine to create the drum tracks for all my recordings, there's also loads of software alternatives out there (loads of info about them in the recordings forum).

The way I see it, any drums I create for one of my recordings exists to give a 'real' drummer an idea of what I want to hear in the full version as & when I play them with a full band.

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#4
I write out drums in Guitar Pro and after doing it for a while, you get pretty good at it. It's gonna sound stupid, but just remember a drummer (usually) has 2 hands and 2 feet, so don't put 5 drums on one beat, lmao. Think of them as fancy metronomes rather than note for note what the drummer should be playing.
#5
I'm still kinda on the fence about this, because another thing I want to be able to do is know a good drummer from a bad one, and I get the feeling it's kinda like how with guitarists if a shitty guitarist plays for some people who don't know the instrument they think he's awesome because he does a few fancy things.
#6
Unless you want to learn drums just for the sake of playing the drums, I'd say no. It will require a lot of time and effort to get to the point where you sound halfway decent -- time that would be better spent playing guitar and writing. You'll have a much easier time just learning how to program basic beats into Guitar Pro or a drum machine.

As far as knowing a good drummer from a bad drummer, there is no right or wrong answer. If you listen to a drummer and you like what you hear (and you feel they mesh well with what you do), then they're a "good" drummer for your situation. It's all completely subjective.