This is one of those "I'm not really sure what I'm asking" type questions.
FINALLY, I found a decent explanation of the BGP update-source command and its usefulness: Neighbors mightn't be directly connected, and therefore there can be alternative paths through the AS to the same neighbor, but arriving on a different interface.
The update-source command also lets you name a loopback address as the update-source (as long as the address is routable through the AS); and apparently, this is the way to go; a loopback interface will never go down.
So...and here's where the not-sure-what-I'm-asking part comes into play...why bother with separate commands here? Why not just set up a loopback address on the routers you want to run BGP and use that address in the neighbor...remote-as command?
Does it make a whole bunch of sense to give an IP address in the neighbor statement, when _all_ of that address's uses are going to be performed instead by a different, update-source address?
I welcome your answers.
Or your interpretation of what my question is.