Well its nice that it can redirect the victims to somewhere else, such as a document explaining that they should not be, where they for some reason thought they were going to go.

... of course you can't use it to block anyone who knows how to disable addons (ctrl+shit+a + disable addon), but you would be surprised how few people know that (especially if you use it against children).
But against anyone with just a basic level of technical knowledge, you should edit the hosts file locally.