Yeah there have been a few apps that have attempted things like this (including Color to an extent).
As for G+ -- another reason that this is so great is that you can make the events private so only the people involved in the even can see the pictures later. And everyone with gmail can then access the photos if they are invited.
While I agree with you that people don't usually use G+, I would say most of the people who came to these parties asked to be invited to the G+ event so they could get @ the pictures or upload their own. They also allow for everyone to download the full size image (in our case ~4MB) while facebook compresses everything you upload.