>If you have enough application servers for session sharing to be a problem, you probably already have enough complexity to worry about
You definitely have lots of complexity. Which is why adding more is bad. There is no point where things are so complex that making it more complex is cool.
> If you don't want to add another daemon to your stack, just reuse whatever you were using before, whether it's Memcached, MongoDB, or plain old MySQL.
Sure, just make things slower and more likely to fail for no reason.
>Gain nothing? Why?
Because you can just store the session in an encrypted cookie. There is no need for it to be on any server at all.