First, it needs mentioning that this functionality has already been removed from the original Bitcoin client.
Here is the GitHub pull request removing this functionality that has been accepted 2 years ago. The wiki page you refer to seems outdated.
The only known use for IP transactions I could find is the one mentioned in the pull request by gmaxwell:
Aww. The removal of this would mean we can't run a lottery for nodes running the most recent version of bitcoin as an incentive to make people upgrade. :)
Another use I can come up with myself would be that you could use the DNS system to create a human-readable address to your IP and use that as your payment address. F.e. I could use
bitcoin.stevenroose.be and put the IP address of the computer running my Bitcoin client in the DNS table for that record. I could then tell people to send bitcoins to that address. However, this method would require a PC to be permanently running and connected to the network in order to be able to receive bitcoins.