Regardless of whether it's just there to advance the plot or not, players will sometimes need to be able to interact with it.
Suppose I introduce a random nameless NPC police officer who's just in the story to deliver a message. (He's obviously a plot device.) A player self-Compels an Aspect to get into a fight with him. Now I need to know his initiative and his combat skills if I want to run the fight. (Fortunately, I can make those up quickly and easily.)
Heh, my answer: "Your uppercut takes the cop by surprise and knocks him out. You now have an unconscious cop at your feet and, probably, a bit of trouble in your future. What are you doing now?"
To me, the point of a compel isn't a random fight, it's the complications that stem from the action. And, if the fight isn't important, I'm not going to waste a lot of time on it.
Same situation here. Unless you're railroading very effectively, your players will occasionally interact with things you didn't expect them to. And then, you have a choice between stats and handwaving everything. The latter won't always work, though it often it'll be just fine.
Nah...they try something they succeed more often than not. (They're heroes after all!) Don't need to put them on tracks to avoid what you call handwaving. Let them succeed! The consequences, even of success, are fun.
So plot devices will need stats from time to time.
PS: By that definition, essentially every NPC is a plot device. And obviously some NPCs should have stats. So why is "plot device" used as a term for statless characters? Because almost nobody uses that definition around here.
Don't think we're using the same definition of "plot device". An NPC can be an ally, a neutral / scenery, an obstacle, an antagonist, or a plot device. Though I tend to dislike NPC plot devices - they often feel too contrived. When it comes down to it, I don't really want an NPC pushing the narrative - rather have the PCs / players doing so.