I think it would be better to use full 256-bits as public key.

What's the point for author to use visible ID?

One reason I can guess is that it would be easier to remember.

Since it is still too long to remember, copy-pasting address will still be the most used way to pass public key.

If we still pass key via copy-past, the shorter id don't save any time. So the shorter(still long) id has no any meaning.

The disadvantages:

Offline wallet become dangerous because there may be chance that you created an id that already existed.

Because you're off-line you didn't know that the id was conflicted. Then you ask someone to send NXT to the id you just generated.

Then you no loner be able to access to the NXT because when you enter the passphrase you were told that the id cannot be used because of the conflict.