It's true: you don't call people with a SIP URL. It would be like dialing on the PSTN with a TID ("Terminal ID", or the actual line address within the switching network). Do you publish a "Skype URL?" No.

Instead, you dial the interoperator code for your provider and "dial through" to the target subscriber. Much like international dialling. And before people criticize it for its "complexity", it's the same as SkypeOut. Except that you can't do it on a closed system such as Skype.

For example, on Pulver's FreeWorldDialup:
http://www.freeworlddialup.com/content/view/full/333/

Sure, the Skype UI did some things well. And the Pulver.communicator does some things well. And other clients do some things well. But if Skype were open and standards based, then the best UI could win. If any other UIs got the sort of proselytization that Skype has had, we'd all be much better off.