Thoughts on multiple registrations to an extension

Disclaimer: I'm not involved in the SIP community and not very  
familiar with its implementation or history.

Whenever I use an IP phone, I run into a nned to register the same  
extension on multiple devices, and I always get the recommendation to  
create more extensions and use hackery on the PBX to make 'em work  
like they're a single one. That's not good enough. The other protocols  
we use to communicate (XMPP, POP/IMAP/SMTP, even proprietary ones like  
AIM and old ones like POTS) are all friendly to multiple connections  
to the same account. Why's this functionality missing from SIP? There  
are only a few use cases, but they're huge ones:

- A user has a desk phone but also wants to take and place calls  
occasionally from a computer or mobile device over cellular data.  
Usually, the desk phone will be the only device registered.
- A user wants an extension represented in two or more geographically  
separate locations, say on a desk phone at home and a desk phone at work
- A house with extensions in several rooms. Right now, the "IP" part  
of IP telephony usually evaporates inside the home, where SIP gets  
bridged to POTS. That's great for retrofits, but as fewer people  
subscribe to landline service, there will come a time when new homes  
aren't wired for POTS. At the same time, many new homes ARE being  
wired with Ethernet.

What do you all think about this problem? (If it's already been  
discussed to death, feel free to lightly whack me over the head and  
point me to the right thread :).)
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