The charger definitely seems to be the problem child of the bunch at the moment, but I don't think it will be phased out of competitive as some people seem to think. I'm basing this mostly on the Japanese Treehouse stream, where a dev (I presume) uses the charger to some pretty good effect.
Skip to one hour. Still a little weaksauce at first sight, but at least better than when I was messing around with it. Fun fact: uncharged shots can still kill.
Someone made the comparison to Team Fortress 2's 6v6 matches, where specialised classes were mostly phased out in favour of classes that can switch roles at will. This makes sense, but I think that Splatoon will be fundamentally different because of some key features: very short respawn time, swimming and super jumps. In TF2 (in my admittedly limited experience), most engagements are fought to the death (unless you play scout) because retreat is difficult and the opponent can follow you quite easily. In Splatoon, thanks to the features I've listed, it's is far more viable to retreat, take cover and regroup once an engagement goes south, giving the enemy a little bit of ground to set yourself up into a more advantageous position about ten seconds later.
And that is why I think that chargers, having to rely on support, can have a role in competitive, because when the players have some good environmental awareness going, you'll never have to fight alone for long. Won't ever need more than one, though. Definitely not more than one if you also have a roller on the team.
Speaking of which, rollers are practically the w+m1 pyro of Splatoon; intimidating at first, but basically a free splat once you've mastered the arcane art of back
pedalling and shooting. Having a roller on the team is important, if only for the huge amount of ink they spread, but just one can that job just fine.
So yeah, in the end, I'd say that, assuming that you're team is heading into an unknown map, you'd want to bring two splatters (Jr. or vanilla depending on personal preference), a charger, and a roller. The splatters make aggressive pushes against the enemy team, the charger locks down areas, and the roller covers the ground. Makes sense to me.