Having the dedicated SFSE community has lots of benefits as Adrian, Dave & Daniel have mentioned & I'm not in favor of moving Q&A back to Stack Overflow.
But having checked out the new documentation site & posted, I don't see why it can't be valuable resource, that sits alongside SFSE. We can build a library of content there, consolidating the knowledge base that's been created in SFSE on core concepts & potentially making it more accessible, while referencing it from SFSE.
I expect that eventually the documentation site will become the first point of reference for many users (assuming it takes off), while the Q&A site will handle edge cases.
I think the concerns in the comments were mainly about the Q&A site but I'll address them in case anyone is worried that they'll also apply to the documentation site.
There's a lot if noise on Stack Overflow that we don't get here, so I don't think getting many users over there is the best case scenario
Judging by how long it's taking for posts to be reviewed, the SO users don't seem to be paying attention to the Salesforce topic's posts so we have the space to shape the documentation.
I think that the separate SFSE community is more effective than having Salesforce topics on general Stack Overflow.
It's seems like a pretty safe bet that the Salesforce topic will form the foundation of SFSE Documentation eventually.
The reason for this sites existence is that a lot of Salesforce specific questions weren't coding/programming related so they would get closed out quickly on the StackOverflow site. That will still be the case, so trying to "move" the community back there would probably leave a lot of users behind, such as our admin friends.
The largest gap in the SFSE knowledge base at the moment seems to be content for admin managed functionality. This may be because admins are already well served by the Success Community, Trailhead & the documentation but there is an opportunity to build a set of resources for admins in the new documentation site, for users who feel a little intimidated about posting admin questions in SFSE, which is obviously more developer orientated.
Having said all that, it is unfortunate that users will end up with two different profiles & sets of reputation.