I can't really say I'm surprised to the response to the question Calipsa asked. But what I am surprised by is why she asked it: she wanted to know if sending group or even guild invites without whispering first was rude, and players responded overwhelmingly that it was. Why did she think it wasn't? There's almost nothing more confusing than receiving a random group or guild invite, and considering that even spammers are using group invites to spam, it's just not something you want to do.
Now maybe she meant just whether a whisper was necessary or not. If a player is marked LFG, or they're a guildie you know is available for a group, or they've just shouted out "LFG for group quests" in a chat channel, then I'd say they're fair game for a ninja invite. You don't need to actually whisper them and confirm everything you do (well, it's still nice to do, but in those cases, I'd say you don't need to).
But yes, invites out of the blue are a no-no. Rude, maybe, annoying, yes, and usually a waste of time for both parties.