In the current situation, NA's do no harm in Octave, but they're also
not much useful, because most functions ignore them, for instance,
mean ([1,2,3,NA]) is NA etc.
On the contrary, inserting checks for NA's everywhere is a good idea
either, because it will slow things down for everyone.
Now that Octave supports OOP, I could imagine an extension package
that provides arrays with missing values as objects,
each object keeping an array and a logical mask indicating missing
values. One could then support lots of operations with these arrays,
that behave "the correct way", instead of the poor support that is
there now.