When the client communicates with the server, how does the server know it's really you? It must be sending something...
That doesn't mean it is a cleartext password.
Account key is stored on the client, and you can use it to login on the project website, so in summary it's the same effect as if email/password was stored on the client. Maybe this was a bigger explanation than you needed, but it's interesting to know

So it's an authentication token. I can handle that.
Why not just set up different accounts? After all, it's your friends' computers, not yours, so you would be getting credits for computers not your own! (I call that cheating, even if I have done it myself).
Because I have about 30 people who would love to participate, but they don't want to set anything up more than a simple "download this, run this, click here, and then leave the rest to me" thing.
Sure, I could set up an account for them, but then I'd still be managing it for them, and honestly it's not worth the headache to have to log into 30 accounts, one per person.
I suppose I could set up a "guest" account myself and let them all sign up to it, and just ask them not to go and change things. Ya, right. That'll work.
Think of me as a sysadmin (who understands a lot about cryptography and how this thing works from a programmer's point of view) having used distributed systems since, oh, 1980 or so.