For me, at least, a real strength of Classic Traveller is its variety of systems for creating content during the course of play - starship encounters, on-world encounters, new worlds, patrons, presence (or not) of the Pisonics Institute - and integrating these into the unfolding situation.
Using someone else's pre-generated starmaps, pre-generated NPC motivations, etc, undercuts this feature of the system.
A contrast, for me, would be Marvel Heroic Roleplaying, or
my LotR adaptation of that system: this is a RPG that benefits from having PCs and situation that evoke and draw on a pre-established fiction.