I could do TCR and TABR with batteries, of course. But then you have to think about that annoying unknown - battery life. Do you have enough charge to get through a night? Do you stop at Hotel X and sleep for 4hrs while recharging even though you're not tired?
I hated riding audaxes with my smaller light and running it low because I was never sure how long it would actually last. With a dynamo I don't think about that any more. It's one less stress I have to deal with which is worth the few watts they cost.