this is a slightly paraphrased question initially posed on the “Web & Web Usability” weblog by Diego Ferreyra:
Imagine every service (any service) provider did that. Blogger, MySpace, Digg, and so on. You would still have 3000 OpenID accounts because none of them would allow you to sign in with the OpenID provider of your choice.
Discussion can come around this: Should the OpenID providers be OpenID providers only?
this may appear an interesting discussion indeed, but an answer to this question seems to be depending upon another one: what goals public Web services want to achieve by incorporating OpenID support?
besides an ‘OpenID Code Bounty‘ which only demands from its participants EITHER to “Implement OpenID 2.0 support as a Relying Party (RP) OR Identity Provider (IdP)”, first, rather obvious answer, which comes to the surface is to provide a deeper market penetration by means of increasing their userbases.
anything else I missed?
PS another nice Diego’s remark from the same blog post:
I wonder what people would’ve said if it was Microsoft who had given this kind of OpenID support.
“Now you can use yourname.passport.com as an OpenID account”, — “No, you can’t sign in into our services using another’s provider OpenID”.
my guess people wouldn’t say anything special, being already accustomed to the ‘One Microsoft Way‘ in particular and a .COM Web services’ common way in general.
JAWW (beta) 
First of all thanks for the quote.
Second: your question “what goals public Web services want to achieve by incorporating OpenID support?” seems to add to the discussion.
The point I was trying to address was, if an OpenID provider provided also a different service, say friends network then to fully *support* OpenID would have to allow others to sign to their services with another’s provider OpenID, which seems contradictory since it’s a “competing” OpenID provider. Maybe OpenID providers should be OpenID providers and that’s it.
The second quote, about Microsoft, I was trying to say (and I think you will agree on this) is that everyone would’ve flamed Microsoft for such a poor support of OpenID, while flowers are being thrown at WordPress for this supossed support.
If you have not seen it yet, I think you will be interested in the conversation around OpenID in the most recent WordPress podcast and its comments.
Well, I’m certainly happy that WP.com is now an OpenID provider. If it’s going to take off, OpenID really needs all the support it can get.
That said, I already have an OpenID (http://bcat.myopenid.com/). I’d love to be able to merge it with my WordPress one somehow. The point of OpenID is to have less identities, not more.
In my humble opinion, a web-site implementing OpenID should become a relying party before it becomes a provider, unless its sole purpose is as a provider.