But, that's the thing. It's not just "window dressing".
Quick, name 3 pre-4e modules that have an eladrin in them, outside of a Planescape module. I can think of one that comes from the tail end of Paizo's Savage Tide AP. But, outside of that? Nothing.
Yet, people are still bothered by the fact that eladrin means grey elf in 4e. But, if you take Planescape out of the equation, all they've done is take some obscure monster that was virtually never used, and repackaged it as a player race. But, since that counters Planescape lore, we have been told that 4e is doing it wrong.
On and on with every single monster. Yugoloths have to hate the gods or they aren't yugoloths. Thing is, in core, yugoloths never hated gods. That's pure Planescape setting canon. Yet, we are going to be repeatedly reminded of this unless we make sure that every single Yugoloth entry in 5e squares with Planescape canon.
By and large, planar monsters in the Monster Manual shouldn't have any more "story text" than any other monster. Why should they? If the Yugoloth's schtick is "They hate gods" then fine, that's a cool enough schtick. But, we must have god hating yugoloths so that they square with Planescape lore isn't fine. That's the problem with Yugoloths as mercenaries. Mercenaries in what fight? Take a look at the original Fiend Folio entry for Mezzodaemon:
That works for me. Now they have a mercenary schtick (note this doesn't apply to daemons, but only to Mezzodaemons) and a pretty decent reason for using them in an adventure.
To me, this is a yugoloth description based on the Great Wheel but completely devoid of any reference to the Blood War.