I would not say that it is necessary to visit each board, but sometimes people focus only on few boards and completely ignore the rest of the forum. Some even put specific boards on ignore, mostly because they consider them as only spam boards. But if you for instance ignore Bitcoin Discussion, you will ignore Important Announcements, and some users would probably save their coins if they read that child board (Electrum phishing warning)...
I agree with you that ignoring specific boards is just too extreme, taking into consideration that you ignore all additional child boards at the same time.
To be honest I never ignored an entire section and don't understand the reasons behind such a decision, especially now when the amounts of spam on the forum are significantly smaller.