I never said to never use Third Eye, you should always use it to mitigate AoE and share etc regardless of situation.
For Seigan usage, it's my personal opinion, but the damage it bring being sparse and spotty is not always worth the investment instead of saving for a early refresh for an add phase, mobility, or a Shinten if my kenki is closer to one.
And like i said at specific time, there's 0 reason to not use it; easy exemple is o1s you'll be able to use Seigan without worry at all because the boss barely move, while for o3s, the boss being more mobile later on or forcing you to dash away, Seigan loose its value in face mobility and being able to stick to the boss as much as possible, you'll still be able to use it but less than possible.