A druid is more like a 1st, 2nd, 3rd, or 4th wheel that you can fit into any of the main party roles. While they lack a few of the utilitarian capacities of a wizard, cleric, or rogue, druids can still do almost anything if they try the right skill, spell, or Wild Shape.
Group lacking a melee specialist? Play a dwarven or gnomish druid with high Constitution and Wisdom, using Wild Shape and spells to be strong in melee. Strength and Dexterity can be average, relying on Wild Shape at later levels and Shillelagh/other spells and racial traits for melee power at low levels. The Animal Companion can also make a great flanking partner and add further to the druid's melee power, especially once the Animal Growth spell is acquired. Spontaneous summons can also help surround an enemy in flanking bashers.
Group lacks a blaster? Druids can rock at blasting stuff to smithereens. Especially with fire. Fire
gooood.

Sure, druids might not be quite up to par with wizards at blasting, but at most levels they should compare reasonably well enough, and they're more survivable than wizards.
Group lacks a healer? Druids make for decent healers, and can be just as effective as clerics in melee while filling that role. Though less efficient and potent at healing, a druid is still better than bards or paladins at the task, and two druids in a party can easily fill two different roles in the party while also serving as emergency healers.
Group needs a skillmonkey/sneak? A druid can handle it. They're less versatile in skills than a rogue but still decent, and they can use their spells for awesome sneakiness, spying, and information gathering. Sure they can't handle traps or locks, but they can form paths around or over them, as well as just plain blasting them or summoning lowly critters to trigger traps harmlessly (or big nasty critters to bash open doors and chests).
A druid can be a fish or a bird, a bear or a snake, a bat or a rat, a horse or a cheetah, a squid or a shark. There's a friggin'
ton of different things a druid can do with the right use of Wild Shape, the right spell, or the right skill.