Originally Posted by
Grimoire-M
It's a case of flavor that the current Lore already justifies to some extent. We are able to access Dreadwyrm Trance and later Demi-Bahamut because of the Calamity scattering Bahamut's Aether all over Eorzea. Phoenix was a Primal that was present during the Calamity, whose Aether was also scattered in a similar manner, though Coil goes into detail how exactly that all worked out. Ergo, since the two are linked, it follows that we have the potential to summon a Demi-Primal version of Phoenix. The Light-suffused Aether of the First just helped us pull that off, somehow, and we get -zero- relevant explanation on that part, but that's the gist. Also, notably, Firebird Trance does deal more damage than Dreadwyrm Trance. By 400 potency or so on single target. I think it's technically less on AoE, but basically the GCD bonus damage given to us from Fountain of Fire/Brand of Purgatory is worth roughly twice that of Deathflare initially and the falloff, while there, isn't a big deal compared to using Phoenix itself.