The Bard, although pretty weak, is the only class that can use the 'Sing' ability. These songs that the Bard sings have a variety of effects from confusing enemies to increasing the party's strength. The most useful song that you will use is probably the Requiem song because it will dish out tremendous damage to undead creatures. The powers and usefulness of the Bard may elude the more aggressive players because of the class' subtleties. Be patient with the Bard; make sure you give the class a chance - get all of the songs, and you won't be disappointed.
The Hunter is good for one thing in our opinion, and that is the Sshot abilty. This skill gives the player the ability to score four consecutive hits in a row! It will take a while to build the Hunter up to the level at which they acquire this ability (a total of 600 ABP), but it is worth it. Start learning it as soon as you can. Also, the Hunters will be useful at this stage of the game with their elemental arrows. The Critter ability, which is the Hunter's default, is fun too watch, but isn't really all that helpful in crucial battles because of its randomness. The Nightingale can help out (it restores about 250 HP per character), but you can't count on it arriving when you need it. All in all, an ok character with a super ability that must be learned as soon as possible!