I think it's a good move pn PC World's part, however...
I'd have expected that Mr Smith would have been smarter than to fall into the old iPhone-bleating trick of saying "it's really another £630".
Like an O2 contract with an iPhone, it's a service. It's a service you'd presumably be planning on using, and paying for, regardless. It does not (as Andy says above) make a MacBook Air cost £1600.
It porbably is a useless service - my own experiences with Three mean they'd have to do some serious bribery to get me to use their service again - but cost of service does not equal cost of product. I know several people who do use Three's current services and actually have very little to complain about - on paper they seem to be trying to compete with T-Mobile on data which can only be a good thing.