I think you just dont understand the basics of the game. Ill repeat: saying that agni is garbage in any team aside from fire is like saying that water is wet. What do experienced DMM players say to newcomers first thing when they are asked advise? Chose your element and stick with it! Saying that only whales can afford multiple element teams is bullshit: 3k jewel draws, premium log in tickets, premium event tickets, and later premium kamihime tickets... ANY player will be able to assemble pure element teams in couple of months if hes VERY unlucky. Ofc i wont put agni in water team but i also wont put amaterasu in water team. Why? Because Jorm wouldve onshot her any time she wouldve been targeted, regardless of how good her abilities are.
This game is not about being a jack of all trades, its about being a professional in one! Once i get 4 fire hime, for my fire team, i will consider swapping any of them for off-element hime only if the candidate is EXTREMELY good for that same reason you described: 80% dmg increase against weak element. Comparing Agni to himes of another element is unreasonable, simply because your goal in the long run is to get pure element teams. Why dont you compare agni to Belphegor while fighting fire boss? Doesnt that seem unfair? Let me ask you this then: would you bring Belphegor instead of Agni while fighting wind boss? I wouldn't.
As for bias, ill repeat myself: im dark/light main, so fire himes dont concern me that much, i just dont understand the logic behind calling a great fire hime "bad" just because shes bad in off-element team. Its like... Duh, no shit bruh. Does that mean shes bad? No. Compare her to all other fire R or SR himes: aside from Brynhilde and maybe Haphestus, shes the best you can get. So by your logic, ALL fire himes that are not brynhilde are trash? Cmon man, thats ridiculous.