In any situation that doesn't involve bulk file transfers, I'd much rather be wired at a guaranteed 100mbps than on WiFi at "up to" 500mbps. (450mbps in my case, since I have an older Airport)
Wired has far lower latency.
Wired isn't subject to interference.
Using a wired connection frees up available WiFi bandwidth for other devices in your home to use.
I doubt any video source we are watching on our Apple TVs exceeds even 20mbps, which is 1/5th of what's available on a 100mbps link. There is absolutely no reason to have a gigabit port on the Apple TV at this point in the product's lifecycle.
Now, that said... gigabit PHYs are a dime a dozen these days, so I'm actually perplexed as to why Apple didn't use one. I know gigabit is more power hungry than 100mbps; maybe they were trying to stay within a tight power budget. Maybe they went the Raspberry PI way and the ethernet interface is connected to the SOC using USB, and they decided to use a 100Mbps interface because it was more available or reliable. Either way, it wasn't a BAD decision at this point.