> 3.x kernels with security patches. What seems to be the issue?

Limiting the scope of vulnerabilities before they are patched.

There's plenty of more recent defence-in-depth work that will make some vulnerabilities impossible to exploit on more recent kernels, and while they will get patched on the older kernels it'd still be nicer if they weren't exploitable in the first place.

Even without that there are lots of power saving fixes that get put into Linux with every release. That kind of stuff seems like it be very important in a phone as well.

Given Android has its own power management story independent of what upstream is doing those power changes are unlikely to be relevant or significant to Android.

I know (vaguely) that android has its own power management system, and I don't know how much it ties into what the kernel uses on a standard laptop.

But the changes to the normal kernel one as well as some of the changes in data structures are algorithms designed to be more efficient in the first place are probably still useful.