I have been thinking about this and what I thought what that if you could play each role to a good enough level, you could understand the game better as a whole. Im not really sure if this is right and if i should focus on all or if i should just keep working on my Mid and my Top.
Learn all positions up to gold or plat level, learn 2 positions like you know the pokemon theme song
It's best to be the Jack of all trades, because it helps handling a variety of champions that way.
I recommend you at least learn a bit of jungling as well. It'll help you better appreciate what the jungler does for you when you're laning. You'll also gain a better understanding of the strategic decisions the jungler makes about when and whether to gank any particular lane.
learn everything if you're low ELOn that way you experience the decision making of every lane. high(er) ELO start having a small champ pool and 2 lanes. (prolly around high hold level)
I think its more important to learn about champions than roles. If you play all the champs, you'll know how to counter them too.
Upto you though , I think its good to know the basics of all the roles, and the specifics / mechanics of your favourites
