>One inherent appeal to a platform like that is that you have one identity across all instances which is appealing for practical reasons (nobody wants to manage a dozen accounts any more)

I see this as a downside. Redditors often stalk others' post and comment histories for flamewar ammunition. This is the means by which politics ends up leaking into unrelated discussions and sub-communities. Putting all of a person's opinions on every possible subject on one publicly-viewable page decreases user privacy, and increases the likelihood of splintered and polarized communities.