I had a little play with expendable AAs and I've noticed the following :-

When I trigger'd one it grey'd out the other expendable AA hotkeys I had setup. Seems only one can be active at any time.

When I trigger'd an expenable AA the total amount of AAs on my toon decreased by the cost of the active expendable AA (i.e. 3). Looks like you will not be able to grind AA count to a silly amount.

An extra 700 hp/mana makes bugger all difference when you get hit for 10k by OOM adds :)

The expendable AAs are an absolute waste of time and energy, unless you have already purchased every AA possible and are thus impossibly uber... in other words "not me." =P

Of course, I missed reading the "expendable" bit in the description and blew away 3 AA. /sigh my bad. And as Seeker says it really makes little difference.