I believe I understand why this question was closed--it seemed to be leading to subjective debate. However:
- UI choices are usually made by the Ayatana Design team, and usually have objectively citable, or researchable reasons.
- LightDM is actually extremely customizable (Wikipedia: "It uses a WebKit backend to draw HTML-based login interfaces."), so it's possible to change the appearance of the login screen without switching to GDM.
The existing answer there was written by two users who later close-voted the question. While answering a question and then voting to close it is appropriate on rare occasion, I think in this case it shows that there's something fundamentally answerable about this question. Future answers could address one or both of the points above.
So I think we should reopen the question.
As an alternative to reopening the question, the OP could be asked to open two separate additional questions:
- One about the design decision.
- The other about customizing the login screen.