[Disclaimer: not an official Google post of any kind, and I might be wrong about some aspect, in which case I just hope someone will correct me.]
Some total stranger just added you to their circles!
Scary! You'd better block them! Right?
Well... no. What does it really mean when this happens?
Does it mean they get access to anything they wouldn't have been able to see otherwise? No. Either way, they can only see your Public posts (or perhaps "Extended Circles", if you actually use that feature).
Does it mean they are creepily following you around? Well, if they see something you post, you're the one who would have marked it Public, so no.
Does it mean they're "friend-requesting" you? No.
Does it mean that the posts they write will appear in your Stream? No. Their posts go to your "Incoming" tab -- which you never have to look at if you don't want to.
Does it mean they are going to spam your "Incoming" tab with, well, spam? Doubtful, and if they try it, they'll get reported and their account will get shut off in pretty short order.
Does it mean that you are obligated to add them to a circle too? I can't even fathom why some people think this so I can't even address it.
What does it mean, then? It means that for whatever reason they'd just like to see your public posts in their stream. Perhaps they feel their stream is too empty, and perhaps they think the public posts you've made so far have been interesting. Is that so bad?
So if all that is true, why would you ever choose to "block" someone? Well, if my posts are appearing in his or her stream, that means they may be commenting on them. IF I start to notice that someone is adding idiotic or offensive or otherwise unpleasant comments on my posts, then I'll be glad for the ability to block. Maybe there would be other reasons, such as if it's someone I know will make such moronic comments :-) but that hasn't come up yet. Or another thing is that people who will comment in a different language from the language of the post are kind of annoying I guess (again I haven't seen it).
I hope this is helpful.