i can't figure how it could harm, hence i'd think of suggesting it for being default....
It harms! Really Moodle builds lots of absolute URLs from this value. And they have to be absolute. So using something like this, you could have, say, half of your content only available from inside your network ('couse your using private non-routable IPs) and the other half only availavle from outside.
The real answer to this problem is using a DNS name that resolves to the right IP each time (the internal one when you are inside your network, and the public one when you are outside it).