Since when? The only time Evoker has been in a Final Fantasy game is III, where it was a lame version of Summoner.
...Before there's any smartass remark, that role in the context of XIV is already taken up by Arcanist.
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...Amazing. I just said III didn't have that type of Evoker (using a summon spell as an evoker gave you a 50/50 shot of either buffing your party or hurting the enemy...except for Titan, who always hurt the enemy, so if anything there was still leaning towards hurting the enemy), and you still used it as an answer. BDFF did, but then it's technically not a Final Fantasy game. (Also it was called Conjurer.)
While it wasn't specializing in, I recall Rydia's Bio being pretty useful in FFIV until you got the -aga spells. If we're going the "tradition" route, then summoner needs to either only use flashy attacks that cost metric loads of MP, since every game with the job system and summoner only had them use Summon Magic, or have them heal - the "White Magic/Summon" combo appeared the most in the series, being in IX, X, and XI.
And yet in other FF games barring ones with an actual job system Summoner has been paired with White or Black Magic. Bio traditionally being a Black Magic spell which somehow is only on Arcanists for some reason.
In 1.0, Bio belonged to Thaumaturge along with Dia, Burn, Choke, etc while Conjurer had all of the elemental spells. Thaumaturge actually had more healing capability paired with DoTs and debuffs. Conjurer was more damage-focused. When the job system was introduced, for some reason SE swapped the roles making Conjurer nearly a pure healer that evolved to White Mage, while Thaumaturge was completely redesigned as a full DPS class to make way for Black Mage.
In the process, I think too much got shuffled and there was no more DoT class because they aligned them to be the base for their new job counterparts.
With ARR, the new class Arcanist resumed that role and inherited Bio as well as the other DoTs. It was certainly an odd choice as a base class for Summoner, but that's how it happened.