I don't know why, but I've run into more than a few ASTs at 60 who insist on giving me Ewers as a BLM. In Creator, even. I don't click it off, because I don't really need to, but I can't stop wondering why.
That said, it got me to thinking about which cards can actively become a disadvantage for BLM. I can't think of any of the effects that would straight up be a disadvantage, except maybe Spear, if it happened to be cast before the BLM starts their rotation. I know my rotation is based entirely around the cooldown times of Ley Lines, Enochian, Raging Strikes, and Convert, and messing those cooldown times up would probably cause me to mess up too.
Also, earlier today I landed in Final Steps with 3 MNKs and an AST. By the time we finished up the adds phase, all 3 MNKs were at around 500 TP each, yet I got a Balance while one of the MNKs received an Arrow. Although to be fair, given this discussion, I suppose it doesn't really matter, and I'm not entirely sure how much Arrow actually benefits me over Balance.
EDIT: I have 1200+ Spell Speed as BLM, and it's only going to get higher because I like having time to dodge AoEs. But the more SpSpeed I get, the less time Arrow takes off my cast time, while Balance is a straight-up gain no matter what. I'm already at the point where Convert is a staple in my rotation because the limiting factor in my Fire IV spam isn't the duration of Enochian, but my MP. When I don't have Convert available during the second Ley Lines/Enochian pop in my rotation (when both Raging Strikes and Convert are on CD), I have enough time to exhaust my MP, hop into Umbral Ice for 1 MP regen tick, then get back to Astral Fire and get one or even two Fire IVs off before getting back to Umbral Ice for Enochian refreshment. So I suppose it is entirely possible that an Ewer, maybe Extended or Enhanced, would be able to regenerate enough MP to negate the need for me to switch into Umbral Ice.
... not that I've been able to test this yet.