It seems a convention in emacs packages to use M-TAB to do symbol
completion. While the default value of flyspell-use-meta-tab is t and
that will override this key binding in various modes such as
AUCTeX. Don't you think flyspell-use-meta-tab default to nil more
optimal?
That would be an improvement in the very specific case you have
mentioned, but it would be inconvenient for all other uses of
Flyspell. All in all, very much a change for the worse.
However, there might be a clever way that these two functionalities
could coexist on M-TAB. If someone can figure out a spec that
would enable both functionalities to coexist, I am sure it could
be implemented. The challenge is to design a good spec that
makes each functionality convenient when you want it.