If this functionality didn't matter to someone then it wouldn't be there. You can say that it doesn't matter to the people you care about, and that's fine. Maybe Ubuntu doesn't want to live in a world where people use directory services for user management. Maybe it's not going to be running in unattended lab environments where you want to be able to configure when the machines go to sleep using the same tools as you'd set up the managed user policy with. Maybe supporting blind people just isn't a concern. But if you're going to make a decision about changing display managers purely because it makes it easier for users to write their own login screens then you should be open about the fact that your goal is more about prettiness than functionality.

For what it's worth, there's no technical problem in making gdm look however you want it to. It's really not that much code. LightDM just makes it easier to start from scratch, even if you then have to eventually reimplement a pile of existing functionality.