I just... really... just read what you wrote few times, because it's... I don't even know how to respond to it.
And that's how Saraswati cut works also, just not for overdrives, but single target attacks. Say you were unlucky and only one Kamihime got hit by mobs, Saraswati can cut further damage received by her by half, effectively saving her life (she can also add 2k heal, just in case).
Also, Snow Raph and Water Osiris cuts are 2T too.
I am not about "win" or "lose" situation here. I just think that whatever argument I will give here you will find a way to say it's not right (after all combo buff 30%/20% for 3T does nothing half of the time). But well, I'll try.
Let's assume my own team, because I know how it works: Herc/Saraswati/Shiva/Atalanta/Triton.
They need 100/90/80/70/60 BG for full burst. Let's ignore incoming damage, it highly depends on content. Assuming worst case scenario this team needs 6/7/6/5/6 turns for FB, it's therefore 2T faster than slowpoke team with Herc. Now, if Saraswati hits at least one combo attack it becomes 6 turns and that's very likely (about 94% chance with no other combo rate modifiers), If you have tad bit more luck you can get to 5T per hime save for last one. Now, if you have really good luck with combo attacks, you can get to 4T. But yeah, in general you can expect to consistently being able to FB in 6T and very often in 5T, that's main power of Saraswati. Note that I almost never use BG up at turn 0, because I find it more useful for adjusting BG later (hitting FB in 5th turn is not that rare).
She has her other uses though, saving someone with 50% cut, saving someone with 2k heal. She's kinda Jack of all trades, she doesn't absolutely excel at one thing, but has her uses at various situations.