I agree with airwalkrr's point. In a marshal, rogue, wizard, cleric party, the marshal is going to march up to the front lines like a fighter and give his buffs, but he is not going to have the damage or control output of a fighter. Instead, the rogue is going to get bonuses to attack, he can enhance the spellcaster's options, his buffs will work on summoned creatures, and so forth. In a fighter, rogue, wizard, marshal party, he can buff like a cleric and also attack in melee, but obviously he is not going to be doing any healing. With a decent Cha he could work some UMD, but mostly that duty is going to fall to the rogue, or the wizard might explore options that add a few healing spells. Or they are just going to spend a lot on potions.
In a traditional four member party, the marshal can stand in, to a degree, for a fighter, cleric, or buff wizard, but not for a skill based character, and cannot be relied on for high damage.
Bards can fill in for rogues, clerics, or wizards (not of the blasting kind, but buffing or hindering), and can pass with poor efficiency for a fighter.
The dragon shaman can fill in for a cleric if they are built right at low levels (and can in general at higher levels), can act as fighters to some extent, and can pass as decent damage dealers. They can work as skill characters mostly for the dragon type class skills.
You could start a party with bards on skills, marshal on front lines, and a dragon shaman healing and blasting with room for one more. There would be no serious deficiencies. All you would need is a trap finder at that point: either a character with trapfinding, or a character with detect magic who could detect magical traps. And by the time you are mid level, one or more of the characters could take Leadership. If the fourth party member were a warlock, this group would virtually never run out of steam.