Why should I login?
No, In the game itself.
In game: so that you can save things online, and comment and vote on saves or add tags to them. On the website: forum, conversations, save comments.
If you don't want to do any of those things, you don't need to login (you can save things locally by using stamps, or holding down the Ctrl key when clicking the save button).
Thanks!
I'm sorry, I know you meant this seriously, and I totally get the question, but still...this is one of of the funniest threads I've seen in a very long time.