Nested functions can be used to make a program easier to read and understand, by cutting down on the amount of explicit parameter passing without introducing lots of global state.
On the other hand, they're not portable to other compilers. (Note compilers, not devices. There aren't many places where gcc doesn't run).
So if you see a place where you can make your program clearer by using a nested function, you have to ask yourself 'Am I optimising for portability or readability'.
