The element <address> contains several elements for marking up the various components of a person/organization's contact information. We have <city>, <street>, <phone>, <email>, etc. - all bits of information about how to contact a person or organization (or whatever the context may be).
However, it seems to me that something is missing: an element for marking up a website URL. Say I am writing an <address> for a company, for example; I might want to include the company's postal address, phone/fax numbers, email address, and website URL. A web URL is, like the other components, sometimes a critical piece of information about how to contact someone.
Initially I had thought about adding something to <address> such as <website>, but it was suggested to me that simply allowing <uri> within <address> might be a better solution, as it would not require the creation of a new element just for this purpose.