Why the Captain isn't a great team player (with others) I went over in some detail during one of the Sharpshooter threads.
Basically I find that he isn't meaty enough or has too little range to be considered an effective wall by me.
When we take a Sharpshooter plus Captain against, let's say, a Berserker plus Gestir scenario, the Captain does not have enough range to get in close to the Gestir, nor can the Sharpshooter help him with that. At the same time, he also will have trouble with the Berserker because he isn't fast enough to catch him, nor can he properly protect himself against his Bigaxe since he can't perfect parry it.
Again, in both cases the Sharpshooter only provides the most basic of support: Damaging the guy that his friend damages. And while the Sharpshooter might be better at burst damage then the Archer, he isn’t at damage over time, meaning the fight will take a bit longer (unless the attackers came in already crippled), and since the Captain is the heavy class with the least health, he tends to retreat or die early.
When the two decide to go for the Sharpshooter, the Captain has trouble helping him. He won't be able to catch a Berserker running past him with his short weapon or get him off of the SS differently. He can catch the Gestir, but can't do anything to actually stop him from attacking the Sharpshooter other than killing him, I guess.
If the Sharpshooter decides to hide behind the Captain, problem number is the case again. He cannot properly deal with two guys when his friends is not helping him getting in close, something an other Captain can help with, partly even the Skirmisher, but not the Sharpshooter.
When we compare this to a Heavy Knight and Archer combo, this will look a whole lot different. The Heavy Knight might be slower in every regard than the Captain, but he has loads of bulk and range. He has no trouble taking on either the Gestir or Berserker in a melee fight by his lonesome. But when taking on two guys you are always at a disadvantage, but fear not, you got your ranged buddy.
The Heavy Knight won't have trouble taking on either the Gestir or Berserker, he does not need any help at melee range at all. Where he does need help is chasing, dealing with ranged classes and retreating. All troubles the Captain doesn't have.
Chasing and dealing with ranged classes will be no problem for an Archer, or any ranged class for that matter, since they have technically an unlimited range. That means you can't run away from them (unless there is cover, of course), and when it comes to dealing with ranged classes, both are on equal ground. Helping with retreating also is no problem because of the Crossbow knock-back.
When it comes to helping the Archer, he can easily hide behind that slab of steel. The Heavy Knight can stay in a battle longer than a Captain, does not require melee ranged support like the Captain, and has an easier time hitting people trying to get around him towards his Archer because of his longer range. When none of these things come into play and everybody is attacking the Heavy Knight, the very basic form of 'Attacking the same guys I'm attacking'-support is all the Heavy Knight needs.
I'd like to mention that I am not trying to say that Heavy Knight or Archer are better than either the Captain or the Sharpshooter, just that they have way better team play. Granted, to have better team play than other teams is a part of the Knights philosophy, but I'd like to see more of that with the Pirates (again, between different classes. Captain supports Captain super duper well)
This lack of a proper wall for the Sharpshooter also led me to say to have him focus more on long range. It will keep him out of the trouble zone, allowing him to support his mateys safely. In mid range he just doesn't get enough support to be effective. Explosives might help, but they tend to stun your buddies.
Of course with the latest patch this might be very well the case. He can reload his rifle while still dodging ranged attack, meaning he doesn't have to go and run behind cover, allowing him to get out a very nice stream of long range rifle shots. And he also isn't useless in Trinket Wars which forces him into mid range, since his pistol is very decent, and he can reload his rifle while still following them to the next battle.
Also, I shall go and grab El Negro to give you a video concerning the charging bar.