Ask HN: obfuscated link vs. username/password?

In building a couple simple web apps, I'm considering using an obfuscated link: http://domain.com/8498j9f43498j9f43o8jeo8je9jfe that members can bookmark, rather than require a username and password every time.

Terrible idea? What are the risks if the url is long and random enough?

I don't sync bookmarks and rarely use them; so take multiple computers/devices and I wouldn't be using your service very easily