He doesn't have abilities to "perfectly fight the main villain". He has one ability to counter one ability of the main villain.
If Oden and kin'emon are literally fighting next to each other, his only contribution is that one ability in that one situation. What if kaido tries to Whip His massive Tail as both Kin'emon and Oden while they together and Only Oden is fast enough to dodge or strong enough to block that. That means he'll have to protect kin'mon on top of himself since nothing kin'emon can do handle a brute force attack from kaido.
It's one ability countering one thing. Brook counters All of Big mom's homies yet If big mom decides to punch him, he dies.
Get it? If Luffy and Brook are side by side fighting big mom, and Big mom chooses not to use a homie and clenchea her fist to punch, BROOK BECOMES A LIABILITY and now Luffy has to not only save himself but Brook as well.
Get it? It's not that hard