Saving as binary - PLEASE??

I posted this question in General Questions on 29th March, and in New Users
on 20th May - no reply from either as yet. It usrely is a basic question for
some of the MVP boffins in he Surely ... someone MUST know the answer??

Here we go:

If binary is more efficient speed and space-wise, why isn't it the default
format (or read "the best way to save a file)?

I'm trying to find out what the downside is before I save a majorly
important, large file with heaps of macros in binary and start moving on
with it to find a little way down the track I've made a terrible mistake! If
it isn't the
default there must be a reason - a down side - but I can't find anything in
Help to tell me.

Anyone know?