I think people might just get annoyed if they don't see why you blocked them. They might find it rude to cut off a communication channel like that. That's probably why it's seen as a "bad" thing. However, there is no problem with blocking someone, you are allowed to do so. Give someone a little time to think about it and they may fully understand why they were blocked. As for the grounds I would go on, I haven't had to yet but I would probably block someone if they were harassing me in some way.
I've only blocked one person, and that's because he was consistently messaging me with things that made me uncomfortable. I told him multiple times to stop, and he continued. That would be reason for me to block anyone else, too.
Blocking someone is the last option for me, yes I won't hesitate to follow that way if it has to be done. I would block someone if:
1. They are outright disrespectful and judgmental towards me and I come to know that while conversing with them.
2. If the don't even try to understand me, then I wouldn't need such people in my life so I would block them.
I have pseudo-moderator powers, block me nya nya nya. I have asperger's too, and i tell everyone what they can and can't post. It's kind of going to suck when you get banned, i like calling you out for your endless supply of retardation.