The ideal composition for a Everquest group was a tank, a healer, a crowd controller, a buffer, and a damage dealer or two. This "perfect group" wasn't exactly easy to get together, depending on which of your friends were available at that time. A party with no healer or crowd control, in particular, was dangerously inefficient. The bard was the perfect class to fill in for any missing roles. No healing class available? A bard could put away his weapons and strum a healing song to help keep everyone alive. No enchanter to mesmerize extra monsters? Let the bard sing a lullaby to put them to sleep. While not the best at any role, a bard could fill in in a pinch and perform adequately. They were Everquest's version of "plug and play".
If flexibility in grouping were all a bard brought to the table, it would be a fantastic character class. But adding to the bard's group-friendliness was the fact that bard songs worked in perfect harmony (if you'll excuse the pun) with all other casters' spells. Typically, similar spells would not "stack" for cumulative effectiveness; a druid's hit point buff would be overridden by a cleric's, for instance. But bard songs were effective on top of any other buffs the party might have. To reach the maximum level of attack speed, armor, or damage dealing capability, you needed a bard in your group. This made bards highly desirable in a raid situation, where dozens of people all had to work together to take down a particular boss monster. Bards kept the main tank highly armored, the clerics full of mana, and the melee attackers stabbing and slashing as fast as possible. A good raid leader wanted a bard in each group in the raid, in order to maximize chances for success.
I don't think it is an exaggeration to say that the bard was the ultimate grouping class in Everquest. They nicely filled in any holes in pick-up groups, and were not redundant even in a well-balanced party. There is much more that could be said about the capabilities of a bard: adept at solo play, able to pull monsters with ease, and my personal favorite, the fastest running speed buff in the game. I had always been a fan of cooperative gameplay, but my years spent in Everquest only strengthened my preference for working together with others to achieve a common goal. No other character class in any game I've played even comes close to the co-op friendliness of Everquest's bard.