Sounds like a dead-on assessment. There are pros and cons.
The nice thing about the Google service APIs is that at least one can code so as to alleviate the #4 concern. In other words, potentially a VineType user could control his/her Google calendar from right within VineType.
Of course, if all that a church (or other group) has done was to simply embed their Google calendar into their site, then yes, they must go to the Google site to make a change before it will show up on their site. But if the church site has been built to take advantage of the appropriate Google service API, they "could" make changes as needed without leaving their site.
Functionally, there is every reason for me to want to log into Google to edit our church's calendar, and I do. They have a nice interface. And since I can (and have) shared edit privileges with other leaders in our church, they can go there and edit too, and they do. I would only want to create a custom approach via their API when the need arises. The latest article on my blog explains why I even wanted to bother with it on that one concern. It’s working great.
I built our church’s site with VB/ASP from the beginning, which was around Jan 2003. (I have still not made the switch to .NET). When I started to build this gadget to interact with Google’s Calendar service, it was surprising to me how few examples were out there that showed me how to do it. I finally was able to piece it together after a day or so, and that’s why I think my VB/ASP code shown on my blog may be of interest to someone. (I know that making it work in .NET may lead to a couple of slight changes, but such a switch-over should not be bad. And the general logic is valuable even if the syntax needs tweaked.)
PS: By the way, although my site for our church is not a VineType powered site, when I recently renovated it, I drew design inspiration from your brilliant design concept on your VineType site. I give credit for that inspiration at the bottom of every page, with a link back to your site. Check it out at http://www.cac.us.com