I could almost see it as worth it for Steam to throw some money at developing support for it for Linux, it would sure surprise Microsoft and Sony if the Steambox could run Mantle.
I guess this is the benefits finally being reaped of getting both major console design wins. Since they already have a large target to support then getting developers on-board should be pretty easy if they give them the tools and are using them anyway on consoles.
Mantle could make a tremendous difference at the low end if it allows slower CPUs to perform adequately in Mantle supported games. The gains may not be as noticeable if you have a modern i5 or greater, since you're probably already getting a consistent >60fps experience. However I assume the minimum FPS situations could be noticeably improved even then.
Simply put, Mantle could make a $500 gaming computer viable in the near future. ~$200 for a decent GPU and then $300 left to spend on all the other parts seems doable if you're building on an AMD platform.