Atreus creator here.

The Atreus was actually originally designed to be complementary to a stationary keyboard like the Kinesis Advantage or the Ergodox. Keep a big keyboard on your desk, and toss a small portable keyboard in your bag for when you're on the go and can't bear the thought of having to fall back to your laptop's internal keyboard.

That said, since I've gotten used to the Atreus I've hardly used my Ergodox; in the end I've found that having all the keys close to the home row is a lot more comfortable, and I don't like having to reach for the edge keys on the Ergodox.