I've made a few complex apps that use Google Spreadsheets as the backend...that is, to hold the public facing data and not, obviously, any proprietary data. This makes it very easy for those who have to maintain the app to enter in data. The downside is, of course, the inability to strongly enforce business rules and to denormalize things...but that forces me to reduce the data design to a bare minimum, which is often the best strategy in the first place.
I hope I never have to be in a situation where I'm building a CMS-type system for a client. People who haven't dealt with data-modeling or relational-databases don't appreciate simplicity...in the end, most people want something that they can type a headline, some text, and attach a photo or two (i.e. a Tumblr). But if you give them reins to design the system, they will inevitably want you to build them something Drupal like. In my experience, I've found that all these different content-relations end up being unused, and the client ends up hacking around them just to get a simple post up.