Whenever I see corporate-initiated projects like this, I wonder where it will be in 5 years. Open-Source projects that have gained popularity more 'organically' like bootstrap (despite the twitter backing!) seem to be a lot more resilient and more likely to be updated and upgraded as time goes on.
Do you really want your SaaS app to look like it was designed in 2015 forever?
I am skeptical that this will be kept up to date when a new manager thinks it's "good enough" and takes the wind out of the sails. Or if someone gets laid off that's in charge of the project. If they were to open source this project it would continue to live on even if the original contributors are no longer involved.
Though I don't see any restriction on where this can be used, it aims for building Salesforce apps.
> What is the Lightning Design System? It is collection of design patterns, components, and guidelines for creating unified UI in the Salesforce ecosystem.
It was also mentioned at the time Microsoft's Office UI Fabric project got released. It is very likely that both projects aim to help third party developers get a consistent look, when developing add-ons for their applications.
Do you really want your SaaS app to look like it was designed in 2015 forever?