That depends on the application. Some applications will run out of stuff to do after a few minutes offline, but others (especially where individual tasks are pretty independent, like Google Docs) can still be very useful for long periods, and can benefit from increased control of caching strategies.
Laptops and desktops still benefit, but less so because generally if you sit down to use one of those devices in an area of poor connectivity, you're going to run out of things you can do with your serviceworker's cached content pretty quickly. If you're developing a serviceworker, the improved experience will be most felt by mobile users.