Every internet site SHOULD include "block member" because most people will get annoyed and bugged by people who spam them every other second with friend messages or threaten them or black mail them. The "block member" button is a great way to prevent thing like this from happening. Also, this way other members can feel like they have enough power and have some control over their account.
Yes, all internet sites should have a "block member" feature as a means to reduce cyberbullying and protect privacy. On too many internet sites, cyberbullies are allowed to roam free and sometimes engage in a practice called "doxxing" in which s/he produces real world information on his/her victim. If the bully had been blocked in the first place, then behavior may not have escalated to "doxxing".