I like this. I'm a parent with multiple kids, and absolutely despise getting Facebook invites to birthday parties where I can't even see the invite because I don't have a Facebook account. Meanwhile every family I know uses Apple these days because the kids need iPads for school, so this finally gives a way of doing birthday/event invites that all families can see/use.
I agree open standards would always be better, but reality doesn't always work like that and instead everyone just uses Facebook "because it's easy". This is now easier, and more open than Facebook at least.