I'm not big on PvE, but from everything I've heard it's something like this.

For tanks: All of them are needed. PLD and WAR are both great tanks that, obviously, every FC will need a few of. DRK, the new tank Job, is a bit bugged and imbalanced atm, and jury is out on how they'll compare to the other two in larger duties.

For healers: Similar to tanks, all of them are needed. WHM and SCH are both great healers that, again similar to tanks, every FC will need a handful of. For AST, the new healer Job, the jury is still out on how they'll compare to the other two in larger duties, and I've heard both good and bad things about the new Job and its performance.

For DPS: NIN is a DPS most FCs will want at least one of for their +10% DMG Taken debuff they can apply to an enemy for 10s every 60s and their new Enmity Control skills. Most FCs will also want a BRD for their songs - recovering MP, lowering enemy magic resistances, and all that good stuff - even if their actual DPS is lower than most other DPS Jobs. If a group has a BRD (which they almost definitely will), they'll likely want a DRG for the Piercing Resistance Reduction debuff and solid DPS. I'm not sure about MNK, but I've heard that they're one of the best single target DPS in the game in sheer numbers. Most FCs will want a BLM for their insane AoE capability - and yes, it's insane even after the recent nerf to Flare. Many FCs will want SMN, too, for their very solid DPS, which apparently has been made even more solid in Heavensward. As for MCH, the new DPS Job, many people are concerned about their current state, as their DPS seems abysmal in most cases - chances are, they'll be buffed; until then, the jury is out.

In short: every Job in existence has reason to be taken into groups and all of them are wonderful (and have their own quirks, too). With the exception of new Jobs, of course, which are still...being learned about by the community.