Append to options

I'm kind of confused - why is it necessary to choose the "other" selection as default in order to append something to the list?

I was hoping if the user was presented with;

choice a
choice b
choice c
other

they could select other, type in choice d, and then it would show up in the list the next time someone added a new node, like this;

choice a
choice b
choice c
choice d
other

I've read posts similar like this one - https://drupal.org/node/1204808

I tried the append option without choosing "other" as default...didn't work. I then chose "other" as default but didn't fill in the blank because the user does not need to (unless they desire) to fill in any part of this list. It wouldn't allow me to save the "other" as default - while leaving a blank text field.

Any other options? No pun intended :)

Thanks