Maybe optimism is profitable on an individual basis. But at the collective level, I am not sure it is such a good idea. Anyway, not being optimistic does not mean you must be pessimistic : We just have to be realistic. That mean, accepting the world for what it is, and not what we want it to be.
That's impossible for anyone in the world.
A pessimist and an optimist will probably consider various interpretations as realistic, yet neither will see the world "as it is".
Even just the idea of the world as something that can be seen, thought of, considered, described etc. is squeezing it through a very human-shaped filter, and what you get out of the other end is always more about us than about the world.