I think its unfortunate that we didnt get the in depth explanation of what the cleaning team actually do and why. maybe its cause im stupid but i dont believe that im the only one that didnt fully get how important the cleaning team actually is plus how the other teams actually think of the cleaning team. There was some explanation on the first few chapters but they didnt really mention of what would happen if the cleaning wasnt done other than the poison and the corpse being an obstacle. the first cleaning operation actually only showed them cleaning the boss room. didnt they have to clean the whole dungeon? why is the view on the cleaning team so bad?
Think about how every single place you go, be it a shopping center, supermarket, general store, restaurant, etc, all have the cleaning team. Now, think of the
absolute mess pigsy place it'll be with even a single day without cleaning, specially if the place is very busy and a lot of people go there.
Now keep that image, and think how people in our world view the people who clean, unfairly seeing them as inferior a lot of the time, even tho cleaning is important and hard work.
Now, pick those two views, and add to that what they clean is monster parts and such that become toxic as they decompose. The hunters will always just see the clean dungeon, and act much like peeps from our world to our mall and restaurant cleaners...
And about the usefulness, as it decomposes more the toxic gas and such gets worse(it's why most of the cleaning scenes mention the density of it and such), so without cleaning, hunters might just die to the poison rather than the monsters, or be unable to attack the dungeon till the gases dissipate and so on. Also why they proposed to have the folks clean while inside for the 24h operation, since it'll be so long the carcasses will start rotting before they leave, which could be bad.
All that doesn't stop them from being bitches to the cleaning folks, tho, much like real life...