As an exploit demonstration, this is at least noteworthy, maybe even interesting. Actually using it is a bit rude though; people set themselves invisible because they want privacy.
Yes, that's the whole point of being invisible. If your friends are misusing a technical feature in a manner you consider rude, perhaps you should try talking to them about it.
What if you discovered a technique to make a phone ring when the ringer is disabled or perhaps even when the phone is switched off? Using it outside of an emergency would obviously be quite rude.
This is a VERY common thing. It means that people tend to not want certain people initiating conversations with them, but they have no problem doing so when they need something.
It's a tool, it doesn't have a correct use case. Just common and uncommon.
My method is to remove people who are offline most of the time, so that they don't get to see my online status in return. I think that online status should be mutual, if I can't see them, they shouldn't be able to see me.