It's good business to be friendly to your users. Also, for people who already use OpenID, having OpenID on your site lowers the barrier for entry.
That's one thing I never got about OpenID and its ilk, though. "We'll fix the way logins work! Just sign up for one more thing and then sites that support this will be accessible." Why is this the cure for sign-ups? Yet one more service to fill out some credentials for when sites do not universally accept it? If the latter part were true, then sure, but in the meantime I don't really see the point.
I am ignorant of the major benefits of OpenID, so if anyone could enlighten me, I'd be happy.
Edit: this post helps http://news.ycombinator.com/item?id=151376
See http://wiki.openid.net/OpenIDServers for a much larger (but nowhere near comprehensive) list of providers.
My use of Ma.gnolia over Del.icio.us illustrates this.
I guess the main idea is that it can't hurt to implement OpenID, it can only help your service. You can still ask your users for all of the data you ask for during a normal registrarion, but /you don't have to/.