why Linux users should care

Yes. Portable software (works the same across all platforms) breaks down barriers to entry for potential Linux users. Firfox, VLC, Thunderbird -- these look and feel the same across all platforms, and it's a HUGE deal.

It means that when a Windows user tries Linux, she can appreciate the performance and usability improvements without having to re-learn how to use her applications. It means that when you leave your Windows computer at work, and fire up your Linux laptop at home, you can use the same apps, the same documents, and even the same preferences.

To put it another way, anything that makes it easier to write portable code makes it more likely to be developed first in Linux, and then ported to other OSes. That is extremely good news for Linux users.