It's not like they're not benefiting from learning the encounter though. I've been watching them an hour or so and they're still making mistakes in P1, which means their members still need time in it to learn the timings of abilities. No reason to not do it now than wait until it's nerfed when you'll have to still learn the timings. As well, Avatar has the benefit that even if you learn one phase you can still learn the rest of the fight despite not being able to down it.
Being able to learn the entirety of an encounter despite not being able to down it is a significant advantage and why so many guilds are still doing it. There's plenty to gain from doing it and I'd say it's hardly going to burn members out in the second week of raiding.