$29 isn't bad. It's more than just wires (unlike display adapters) since it runs off the Thunderbolt bus. A HDMI ethernet adapter could be just wires, but that would require an ethernet controller in the computer, and being limited to 100mbps would not be ideal.
The fact that it's on the Thunderbolt bus doesn't make it faster than built-in ethernet. The ethernet standard is still the limitation, and built-in ethernet runs on the PCI Express bus, which is basically the same as Thunderbolt without the extra expense and marketing hype.
My internet connection is faster than 100mbps, and I've seen speeds in the hundreds of megabits at off peak times. Wifi is a nice idea, but the spectrum is polluted by too much traffic in the building I'm in. 5GHz used to be quiet, but now they are putting 5GHz into the building, so that will get polluted too (and it won't fix all the problems; the routers only have a gigabit connection which is shared by all the users on the router). It's also nice to have fast network transfers, and I've done full backups between my PC and MBP over gigabit.
The fact that it's on the Thunderbolt bus doesn't make it faster than built-in ethernet. The ethernet standard is still the limitation, and built-in ethernet runs on the PCI Express bus, which is basically the same as Thunderbolt without the extra expense and marketing hype.
My internet connection is faster than 100mbps, and I've seen speeds in the hundreds of megabits at off peak times. Wifi is a nice idea, but the spectrum is polluted by too much traffic in the building I'm in. 5GHz used to be quiet, but now they are putting 5GHz into the building, so that will get polluted too (and it won't fix all the problems; the routers only have a gigabit connection which is shared by all the users on the router). It's also nice to have fast network transfers, and I've done full backups between my PC and MBP over gigabit.