Simpler, but more expensive: in a pure Thunderbolt device like the Cinema Display, to support USB 2.0, FireWire 800, Ethernet, audio, etc, there must be controller chips for each on the Thunderbolt device itself.
It's not acting as a hub in a classic sense, "extending" those connectors from the laptop -- it's duplicating those types of connectors by including a full additional controller for each, and connecting them all to the laptop via PCI-e.
To put it another way: it's not acting like half-a-dozen extension cables all at once; it's acting like half-a-dozen expansion cards at once, because (for all intents and purposes) it is.
It's an elegantly versatile way of handling the age-old docking station problem, but it comes with a high price tag, because it means the docking station needs to be half a computer all on its own. For something like the MacBook Air, which doesn't include it's own Firewire or Ethernet controllers to begin with, that makes a lot of sense. But you can understand why PC manufacturers might want to save costs by avoiding some of that unnecessary duplication, which is presumably one of the features of this extended port.
(But then you're halfway back to the problems always posed by proprietary docking ports. Personally, I doubt this semi-TB docking system will really take off. In the long run, the costs of controller chips for Apple's approach isn't that much, and the resulting benefits in versatility are probably worth it.)
