Having two Ethernet ports makes running the LAN->WAN router use case much simpler. With a thin client with one port you can run "one armed" with a managed switch, or use USB-Ethernet adapters like many do with Raspberry Pi.
The T620 does look quite good on energy usage. If you already have one it could certainly be used. If you're going to buy something look for a mini PC with two or more Ethernet ports.