frankly, tracking seems to be a primary purpose of the portal tag. it takes the imperative away from the user so that the server can make the decision about what you see and how intrusive it can be. the point seems to be to shift control to google.
I honestly don't see how the server gets any more "power" than it already did.
We already have enough issues with tracking pixels, but now we'll have to worry about multiple sites worth of trackers (which I'm sure google will do their absolute best to obfuscate behind the background network call and other sandboxing shenanigans)? No thanks.
(and you can be sure that anyone that dosen't allow google to "portal" their pages will be damnatio memoriae'd into oblivion on results pages)