Security and possibly performance, which is the selling point of MV3.
Saying anything positive about MV3 or the lite extension seems to get you downvoted without explanation though, which is a nice example of how absurd this site is when it comes to anything related to Google.
Sometimes I think downvoting should require leaving a comment and reason, because I can't see any reason to downvote this other than "google bad".
This is not true.
People talk about the upside of the declarative API plenty, but adding one function doesn't mean removing another, and the conflation required to use that as a defense of google is what gets downvotes.
We aren’t talking just an extension here. If it didn’t exist, that would make web browsing insufferable to many. It is a part of web browsing itself. Let me put it as clearly as it can be:
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uBO is a Holy Grail and gorhill is our Jesus Christ.
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If MV3 (and further development) tries to touch it in any inappropriate way, comments promoting it deserve 5x downvote mutiplier without the usual -4 limit.