The more the Linux kernel is used, the more contributions and bug fixes it will have from companies that use it. This might also mean better hardware support -- which we all benefit from.
Exactly, so if the shell is a bunch of non-free restricted crap or is under the control of someone else in a remote server... what's the point? What practical difference does it make to a user to be running Android or iOS? What difference does it make to them if there is an invisible Linux kernel underneath?
It's no coincidence that six out of ten in this list run on Linux:
http://en.wikipedia.org/wiki/Comparison_of_mobile_operating_...
In a roundabout way it may actually matter to users in that it enables more competition and better choice.