I think it's dangerous because of the dependence on the client, and useless because scrypt(password,salt,work_factor) on the server is plenty hard to attack.
For a large enough work_factor. In practice "large enough" is usually interpreted to mean "something that seems safe without being so large that I need to buy lots more servers"
The argument (which I'm interested in, but not yet sold on) is that moving the scrypt to the client allows you to pump up the work_factor even higher than you would have been willing to do on the server.
In general though, pushing auth down to clients in Javascript makes my skin crawl. You're one XSS away from having an attacker no-op your scrypt and return SHA256("secret-attacker-password") on registration. The logical follow-up to that: 'have the server run scrypt the first time' -- but then you've just moved the tough work to user registration, which seems just as exploitable.
I dunno. Safe crypto is hard enough already; I'm not sure pushing it into Javascript on the browser makes it any easier.
The DOS risk on registration can be mitigated more easily than the login one. e.g. Rate limiting new registrations will often be more palatable than limiting logins, or you can require "email validation" before you set the first password. And, not every application allows self registration.
It enables DoS attacks, though, right?