Doug McCrae said:As you say yourself, it's a plot device, not a monster.
I remember seeing some of the plot devices masquerading as monsters in earlier editions and thinking "What is this doing in the same section as the things you hack to bits?" That sort of creature doesn't need an initiative score, armor class, etc.
Sure it does, when the PCs finally figure out what it's up to and attack it.

Seriously, giving it mechanics serves a purpose. How good a liar is it? If it's with the PCs and they're hit by a fireball, does it die? How fast can it travel -- will it slow down the PCs on a long march, or not? Can it help carry the treasure? Etc. There's plenty of reasons for statting out such creatures. Maybe they need a different 'template' than monsters, or should be handled differently, but treating them as purely statless incarnations of DM fiat isn't the way to go, esp. in games like D&D.