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.
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!
Things with strings:
Ibanez J.Custom, Prestiges, RG8, SR5 bass etc
LP's, Strat, Tele
ENGL Retro Tube 50
5150 III 50W
Orange Terror Bass
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.
Gibson LP Traditional, LP GT, LP Studio, SG Standard x2
Barber Tone Press > EHX Worm >TC Polytune > EXH Glove > EHX East River Drive > Zoom G3 > TC Spark Mini Booster
Jet City JCA22H
My SoundCloud
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.
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.
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.