Ron Hunter wrote:

That spare will still cost many times what a set of Eneloop (or similar)
batteries,

Actually it won't. A set of six Eneloops for a digital SLR vertical grip
would cost about the same as a Li-Ion for the same camera. I.e. a
2000mAH, 7.4V BP511 costs around $12. Six 2000mAH 1.2V Eneloops for the
vertical grip would cost around $14, and would provide the same WH.

The difference is that you can go to Smarty-Mart and buy those Eneloops
for $2.25 to $2.50 each, but to buy a decent aftermarket BP-511 for $12
you have to order it from an online source. This is one advantage of AA
batteries versus Li-Ion.

Some real advantages of AA batteries a

1. Li-Ion packs are proprietary, you can't substitute disposable AA
cells if your battery goes dead in the middle of nowhere, and you have
no spare battery, and no AC or DC power for charging
2. Longer shelf life
3. Faster charging
4. AA cells will always be available, while less popular Li-Ion packs
may be discontinued
5. Multiple devices that use AA or AAA cells can share batteries and
chargers (though there are now Li-Ion chargers that can charge many
different battery types)
6. NiMH AA cells can be charged from a 5 volt USB port, while larger
Li-Ion packs (7.4V) cannot.
7. Hot Shoe Flash and Camera Can Use the Same Type of Battery

and will you still be able to buy a battery for that camera
10 years from now?

Batteries for the very popular digital SLRs will easily be available for
ten years. For second tier digital SLRs they'll probably still be
available, but not at low prices.

It's rather moot since none of the good digital SLRs use AA batteries
except in the vertical grip.