Trust me, we have had plenty of debates over dynamos vs batteries on this forum. Just do a search if you are interested.
Regarding AA batteries, I can testify that Eneloops are incredible. My Dinotte 140 taillight uses AA batteries, and I have tried various brands over the years. Eneloops are my favorites by far. Altho not rated as high in raw power as some AAs, they hold a charge better than any other AAs that I've used. Plus, they hold their charge while stored.
There are advantages for headlights using AA batteries. In addition to their low cost vs lithiums, you can easily carry extra batteries to extend run time or for emergency backups. Almost anyone who uses lights regularly for commuting will eventually forget to charge their batteries and run low. With AA batteries, particularly Eneloops with their long storage time, you can easily carry a set of spares in your seatbag.
Lithium batteries also can catch fire or explode in rare circumstances.