Anyone got video of this in action?
Knowing the alternative tuigreet ( https://github.com/apognu/tuigreet ), there won't be much of a difference between a video and the given screenshot. You have a prompt, you enter username and password and your Wayland compositor/X session starts up.
The thing display/login managers have going for them is they allow certain configuration before logging in, or they show certain information before logging in. Whatever you want to configure or display depends (mostly on preference). Thinking about it, I am not sure I need anything. I mean, all I do is type sway.