I'm not sure how many CORs are "bad" but I'd guess it's a significant percentage. I've seen ones that almost never shoot, ones that don't use QD to enhance enfeebs properly, ones who don't double-up on 6. EVER! I've seen CORs who don't have /dnc or don't have /mage. Seen CORs who have /mage and use /mage but don't have any attempt at an MP build, therefore rendering their support heal capabilities almost non-existent.
Even our rolls, which are (aside from relic head) completely unrelated to gear, are vastly vastly improved by merit abilities. I know many CORs who aren't 20/20. If if you've got that, having some COR type combat skill merits (particularly marksmanship) can take our COR up a notch too.
And I haven't even talked about the difference between a COR who puts in a ton of effort at events to get strong gear vs. those who don't, can't or just rely on AH gear.
All those things make a huge difference in a COR's performance. A COR can impact a pt or event in so many different ways (knowledge of when to use which roll is another key aspect some don't have). My Nyzul group once defeated a Cerberus boss with white magic restricted lol. We thought we were screwed but I knew how much easier he is when the melees have Magus n MNK roll. Cerb didn't gain much tp and when he did, his fire-based stuff didn't do much damage at all. A lot of CORs don't even know that those are two of the most powerful rolls that we have.
Quoted for truth. These are hella powerful and I have never seen corsairs use these. When they are used, people call you noob and laugh at you.
Corsair roll is good for merit party, even though sam or fighter roll adds more dot to a kill, this roll alone would beat those rolls in exp per hour as the mobs are cleared anyways.
Magus roll should be stacked when fighting heavy magic mobs and is a must when fighting AV or at least attempting to hold him as much as possible before finally dying.
Monk roll is good on salvage bosses, sandworm serket, cerberus, khimaria, the list goes on.
beast roll is deadly for summoners and garuda especially.
drachen roll is a ridiculous roll for an all dragoon party.
Warlocks roll is actually better then wizards roll on most HNMs. I don't see anyone ever using this roll on blms.
Sch roll: read warlocks roll. This is sometimes better in low man blm events where the blm needs as much MP per nuke that they can get along with a refresh roll which is a must.
The only sh*tty roll a cor has is the pld roll. the other rolls are godly and should never be laughed at for any reason. I do use the pld roll on plds that are being hit by a mob that is using hundred fists though then I change it out.
Earth shot is very underrated. Anyone uses earth shot over thunder shot is considered a noob by many.
Earth shot adds extra slow % to the slow spell. I think its way better then thundershot.