Puts a lot of work on the GM in terms of bringing someting to the table if they've already decided how X, Y, and Z works, but at the same time, having any published material, meta-plot or not, can have the same effect if you've already taken a part of the campaign setting for yourself and it's all of the sudden detailed out.
There is a difference in kind, though: If I'm filling in something that wasn't previously detailed, I can fully expect that what I'm doing may get contradicted by later supplements. But since both me and the supplements are both building on the same foundation, there's a pretty good chance that whatever gets published will, in fact, be fairly compatible with whatever I've created on my own initiative.
Metaplot, on the other hand, goes after the "safe zones" and changes them.
Imagine if the campaign world described a house: If I were to start using that house in my campaign, I might redecorate the living room. I might even start building a new addition with a couple of extra rooms on the back.
If a new supplement comes along and describes that house in more detail, I might discover that they used a different couch. Or maybe they've put a swimming pool in the backyard where I was building my addition. These can be problematic, but I can shift the swimming pool into the side yard. And, heck, what's the difference between one couch and another, right? I can either add a second couch in the living room or ask the NPCs described in the supplement to sit in my couch instead. I can still generally get usefulness out of the supplement.
Drastic changes wrought by metaplot, OTOH, tend to be more like: "The house has exploded." "The house has been invaded by a roving gang of rapists." "The top of the house has been razed away by a cloud giant." Either I change my house to fit the radical re-design; or all future descriptions of the house are useless to me and I miss out on the nifty swimming pools.
(Perhaps I've stretched this metaphor too far.)