Prism
Explorer
I don't remember the adventure suggesting that none of your assets matter. In fact I'm pretty sure there is a list of groups for your assets to counter. Which means without them the final encounter should be more difficult. If you felt that your assets didn't matter I the final battle... That's your DMs fault.
I'm now here's near this part of the adventure but I have a few ideas about how/when to apply the heroes assets... But given that it's up to the heroes all my plans/ideas will probably b out the window.
Yeah the forces of good are vital in the last battle. Without them the encounter becomes nearly impossible as extra forces are arrayed directly against the party. The forces can also be assigned tasks which directly interfere with the summoning of Tiamat and reduce her combat ability. The module describes a few examples of this but suggests that its up to the party on how to best use their allies.