Some notes are missing from this, the gateway can handle up to 4 devices at a time (depending on what bandwidth you have), when plugged in the first time it will perform bandwidth tests.
As for data, well not all phones have wifi access but the idea of this is that you can still make and recieve phone calls when you are at home (and a lot of buildings have terrible construction that blocks signals), so this is a bonus for those customers that want people to still call them when they are at home or in the sticks (obviously if they have broadband there too).
It's designed to compliment the coverage that they provide, remember when planning permissions are submitted to get masts put up a lot of people reject it and then complain later that they cant make phone calls...
Also the device only works with phones you register yourself (by entering the mobile number on a setup page on the Vodafone website) so it's not like anyone will leech your bandwidth, and as calls go over a GSM connection which is slower connection then even GPRS I can't imagine a voice call taking up huge amounts of bandwidth, can you?
And for those that ask why on earth you want to have one when your phone works at home then its not for you, I can see a lot of people with soho offices at home that could do with one of these.