In my conversion of "Dragons of Despair" I wrote that players are "encouraged" to use the pregenerated PCs. I made that assertion based partly on the fact that I do believe the backstories of those heroes add depth the established overarching plot of the adventure arc, and partly out of tradition: that's what the Dragonlance modules have always advised.
In addition, suggesting that players use the pregenned PCs justifies dedicating wordcount to dramatic moments or plot points that feature the backstories of those characters—for example, Kitiara's letter in DL1. The real purpose of Kitiara's letter in the adventure is threefold: 1) to foreshadow her later involvement as a villain, 2) to establish her relationship to some of the heroes, namely Tanis, Raistlin, and Caramon; and 3) to urge the PCs to move out of Solace quickly. Items 1 and becomes meaningless if the pregenned PCs aren't being used; and item 3 isn't critical in the grand scheme of things.
The truth is I think the Dragonlance modules run just fine with any original group of heroes the players come up with. In what amounts to a contradiction to my "bottom-up" approach to the adventure, I did include a short appendix in DL1 that makes suggestions regarding setting-appropriate race and class selections for PCs. I included these suggestions out of respect for the setting’s traditions, but I also think many groups would not be concerned about what would or would not be appropriate in the Dragonlance setting—they just want to have fun playing the game. In such a group you could easily have a warforged fighter, an eladrin druid, a dwarf invoker, a shifter ranger, a half-orc rogue, a deva avenger, a dragonborn paladin, and a goliath warden. Instead of a staff of blue crystal, maybe the paladin has come into possession of the Blue Crystal Sword. It would make absolutely no sense in the context of Dragonlance, but the adventure would still be great fun, and the players are playing the heroes they want to play.
The 3E adaptation of the Dragonlance modules published by Margaret Weis Productions addressed these issues by using a really clever device that they called character archetypes. These identified the archetypal roles played in the adventure by each of the pregenerated heroes. For example, Tanis is the Leader, Goldmoon is the Prophet, and Raistlin is the Sage. The idea is that a group can map its own PCs to those same roles, and then dramatic story-based moments in the adventure that originally applied to the classic heroes can be easily transferred to the player's own heroes.
I thought about doing something similar in my conversion but ultimately decided not to, simply because I felt that if the players used the pregenerated heroes, there would be built-in roleplaying hooks in the adventure for them; and if they used their own characters, they could provide their own hooks and sources of drama. I thought that any plot points "requiring" a certain type of character could be subtly reworked so that the archetype isn't truly necessary in the end.
In DL1, this is best illustrated with the example of Goldmoon. At first glance the adventure needs a cleric—someone to wield the Blue Crystal Staff and to become the first post-Cataclysmic cleric of Mishakal. But really, in 4E, any divine character would do. To take it even further, you don't need a divine character at all—the PCs really just need to get the Staff to Xak Tsaroth to hear the message of the goddess, and then they can use it to help them defeat Khisanth. If there's no PC to become a cleric (or invoker, or avenger, or whatever) at the end of the adventure, that's fine. They're going to take the knowledge of the True Gods to the NPC Elistan in the next adventure anyway.
Handling it this way gives the story perhaps less dramatic heft than it would have if the classic heroes were in use—but a group preferring to use its own original heroes will provide plenty of their own dramatic moments and introduce their own themes if they’re dedicated roleplayers. Some groups will want to “play the novels” and other groups will want to “play the modules.” Both groups will have fun, and I wanted the converted module to be accessible to both playstyles.
And that’s why I dedicated a lot of space to the character sheets for the classic heroes, and interspersed the adventure with references to their dramatic stories, but also included the appendix about original characters. I think for Dragonlance, it’s just fine if you bring your own heroes.