I definitely don't want my email application looking at my contacts' contacts, though. That would leak information. And would only work within the same provider.
Who's privacy is being violated if a friend of a friend invites you to do something?
The system in my mind wouldn't tell contacts of contacts "hey, did you know you can invite this person that you've never met with the email@gmail.com that you were previously unaware of (and knows Susy and John) via google calendar?"
It would just whitelist contacts of contacts, and would probably cut out 99.9% of the spam with little to no impact on the user.
It's much less intrusive than Facebook saying "hey, these two friends of yours know this person who is not your friend, do you know them?", at least.
Let's say I'm in the closet, and GLAAD sends out an invite that lands on my father's calendar. Hmmm, who among his contacts also has GLAAD in theirs?
I'm sure there are other scenarios. I don't want my contacts list being used to filter email for other people in my contacts list. It's my list. Not a public web-of-trust thing.
Alice makes a new account and adds Bob to her contacts. Alice sends herself many invitations with spoofed From headers. The ones that get through are Bob's contacts. Bob's privacy was violated.
I personally really appreciate having stuff like restaurant reservations, trains, planes, hotels etc being automatically added to my calendar (which tend to come from no-reply addresses).
I also haven't had seen any spam invites, presumably since Apple's thing is smart enough to ignore email in the spam folder?
That's insanely dumb.
Why not at least limit calendar invites to contacts or contacts of contacts?