It is needed not just for battleships so you can see your shells land. It is also needed
for Heavy Cruisers and high level light cruisers because those ships have some range too,
and you will need to see those shells land so you can play effectively.

Thanks. I'm on a BB and nice guide.

It depends on what you are driving. Experts and vets are needed for sight range,
guidelines and the BOs ability to restore and repair.

For SS, the one who gives sight is the Sonarman, so no experts or vets or even
rookies are needed in the BO.

For CVs, you are mainly playing with planes. Experts and vets will help with your
sight range, restore and repair ability... they are somewhat needed in this ship class
depending if you can reach 900SD with other sailors.

For BBs, they are needed and are vital for you to be able to see the shells land. It is
usefull for lower levels and while in higher levels you will be able to see farther than
the shells, they help your BO weight less. In higher levels you will always need to
trim your BO to get your guidelines in line with the landing of the shells. The amount
of experts and vets you have will determine how much trimming you will need to do
for that to happen.