If Katakuri were at the Rooftop instead of useless Killer and featless Kidd, he would have contributed to the fight much more than them, like literally.
Let's see what Killer and Kidd did on the rooftop and compare, shall we?
Kidd:
1) Tanked a CoA-less and ACoC-less smash from Big Mom. Nothing impressive considered there was like no Haki in it.
2) Shot some pathetic metal rods at big mom that does nothing.
3) Punk attack gave dragon Kaido a nice internal massage that dealt like no damage.
4) Tanked an AoE attack from Big mom that uh wasn't impressive honestly, like it was such a far away shot, and that attack almost destroyed nothing on Onigashima, idk what AP is that attack even. Idk if you can even say he tanked it since that attack had such a huge AoE and he was far from the fire-point.
5) Caught Big mom offguard by pushing her face into the ground, but said attack did nothing to Big mom so it is pointless.
6) Sent big mom flying to the sky with magnetism, but doesn't show much since he dealt no damage.
Killer:
1) Chipped Kaido with his internal attack once.
2) Chased cut Zeus and chased Napoleon with Law's assistance
3) Tanked the unknown AoE attack from big mom.
4) Got stalled by Hawkins
Now let's see what Katakuri can do
1) No sell CoA-less and ACoC-less attack from BIg mom because he is Logia.
2) With Law's assistance can trap Zeus and throw Big Mom off the rooftop simultaneously with Awakening.
3) No-sells that AoE attack because he is logia.
4) Can give Kaido similar internal massage via compression using his big Mochi arms (I mean Killer literally tried to compress Kaido, Katakuri can literally do the same), Kata can probably even add fire to his massage to deal "internal damage". Maybe even attempt to suffocate Kaido( unlikely to work though obviously, but still better than what Kidd and Killer can do).
5) Would easily destroy Hawkins.
In addition to :
1) Be a much better assist to the rooftop Luffy, Law and Zoro via his mochi powers, such as providing some Mochi defense in addition to Zoro's, or moving his teammates around with his Mochi (very helpful to Law).
2) Distract Big mom and kaido via Mochi
3) Future sight helps team to coordinate better.
4) Can probably even throw Big mom /Kaido off the roof if they get too cocky. Hybrid form Kaido doesn't fly anyways.
See, Katakuri alone is much more valuable as a teammate than Kidd and Killer combined on the rooftop. He can easily do what both of them have done on the rooftop (which is quite frankly, uh nothing much) up till now, and contribute even further with the versatility of his Mochi powers.
Just because Blackbeard's crew "killed" WB doesn't mean they all scale to him.