If the numeric stayed as a "hidden" field for WA's purposes and the user id was public, it would be better for people using the user id to sign in. It is a lot easier to use your standard user id than to remember a new number - especially when it has no significance to you. (and less typing for an id as opposed to an email - plus email changes more frequently as providers merge, etc)
Having the user id be unique within the system is good.
Having it be modifiable - nice, but not necessary . . . It would get used by people that used a name that they then changed via marriage, etc - but my experience on my current site and the associated public forum is that I've only had 2 people in 5 years that asked to change their ids. It could not be done on the system - had to delete one and create a new one . . . so it would be nice to be modified, but not a high priority.