Unnecessary is very subjective. Something unnecessary for one team member may be crucial to another. One might say that the cost of specialization and focus is a growing amount of time not spent on one's area of specialization and focus.

Communication overhead increases as the number of people increases. Due to combinatorial explosion, the number of different communication channels increases rapidly with the number of people.[3] Everyone working on the same task needs to keep in sync, so as more people are added they spend more time trying to find out what everyone else is doing.

https://en.wikipedia.org/wiki/The_Mythical_Man-Month#Communi...