Hussar
Legend
There is a plane called Mechanus. It would be pretty embarrassing if the game failed to answer the question of who lives there.
Even if you think your players won't fight them, that's not a good reason to omit them.
I thought worldbuilding was the entire point of a monster manual. What do you think the point of a monster manual is?
The point of the monster manual is to provide interesting things for the PC's to interact with. As a world building tool, it's an abysmal failure.
This was precisely the same sort of discussion that happened when the 4e Dryad was first unveiled. The pre-4e Dryad is essentially an Aha-Gotcha monster. If it gets into a fight, it charms. If the charm fails, it dies. It cannot really fight, has no real offensive power and is largely just a trap.
The 4e Dryad, OTOH, can do everything that the 3e and earlier dryad can do. It can interact with the party and try to seduce them or whatever it is the dryad wants to do. But, if the fight starts, it's also capable of being more than a saving throw away from being a speedbump. In 3e, it's a CR 3 creature because of its charm ability, but has 14 hp. If the charm fails, it dies.
This is the same problem I have with something like Modrons. Inscrutable beings with indecipherable goals make for really bad games. Compare them to Formians - invasive insectoid borg beings who plop down and begin assimilating everything around them. Now that's something I can drop into the game without any real problem. They come pre-packaged with lots of conflict.
Can you make modrons and slaad interesting? Sure. Celebrim has proven that in this thread. You can add all sorts of stuff to make them interesting. To me, it doesn't matter though. A well designed creature in the Monster Manual shouldn't need me to add an entire setting just to make them interesting. They should be interesting in a paragraph.