Spreading good could get into a philosophical argument about how "good" it is to shoulder out indigenous varieties of plants and animals with good aligned versions as the sphere of influence grows.
I personally kinda see good biomes in the same way as the Hallow from Terraria. It's the "good" equivalent, except its still not particularly good (I suppose a better way to describe it would be "fey") - its just as dangerous as the evil regions (atleast in Terraria, in DF, maybe not so much since the good regions are less fleshed out, unless you use mods), what with unicorns freely goring people to death and the like. So spreading good biomes would still not actually be the good thing to do.
I mean if there's always some potentional perverse benefit to doing so you might want to, supernaturally ideosyncratic 'demons of light' and other light magic associated entities like that being more of a annoyance than primordial unicorns are potentially. Some benefits like extension of your life for the longetivity sphere but the areas being patrolled by bloodthirsty magical sentinels.
Others might contextually be annoying, like a sphere of music making insufferable sound which makes it hard to sleep,
there's a bunch of suggestions of spheres here.Will there be sphere created semi/mega-beasts where appropriate emerging with the wildlife (within world costing), or will they always be resorted to world generation?I was kind of hoping to have a
greater muck monster for the evil biomes pop up when the conditions were right, or inversely to the quotation, maybe a Phoenix spontaneously appear over rebirth good areas to add some danger setting things on fire with the halves of a roc and a dragon together.
Will certain creatures or entities be able to spread good regions?
I think that the answer to this is no.
Without a proper reply from Toady, until its all sorted out, i think evil biomes as they are will just be 'Primordial' until least after the changes to world generation are put in and a clean slate to make the world as they want it. But i mirror your opinion that spreading good biomes at this stage doesn't really have a purpose outside of being there for modders.
Potentially some interaction with angel vaults or shrines I abstractly think of having a higher potential chance of hosting some couter or alternative influence but thats just my own speculation.