What it is the value of WYSIWIG?
It provides immediacy and the ability to fiddle quickly. Bret Victor capures it perfectly in this video where he shows the importance of immediacy in the creative process during the coding of animations and games. Creativity benefits hugely from experimentation, playing and being in the moment. You can't achieve that by analysis, structure and prethought.
WYSIWYG editors have allowed millions of people to access functionality which would be out of reach otherwise. Also if people can see the results of what they have done straight away they can learn quickly how to change it.
But why is this an either or... why can't you provide access to the WYSIWIG AND some structure?
I have to say that I love using a Wordpress which I think is a hybrid (some WYSIWIG some structure)... they have done such a great job of seperating the presentation layer from the content.
It allows one to edit the text easily in in a simple visual editor (it is basically just WYSIWIG of the html elements not the css) on one tab and then fiddle with the html on another. It also lets you add CSS style sheets to customize the themes or start over and just use the CMS layer for managing the content.
The great thing is that each user can find their own level. It also allows them to grow and learn. Take the steps when they are ready i.e. access is not restricted by the tool.
I have used Wordpress with so many clients who just want a simple way to be able to edit their sites on a regular basis. I have taught children to use it.
Some of them have gone on to learn how to edit the CSS ... by never hiding the html ... it has provided them entrances to the next level up.
BUT if I had my way we would go even more extreme WYSIWIG
My one bug bear with Wordpress is that you can't see changes live... so it is a pain to "fiddle". You have to update and then see changes. Also the CSS is too hidden and can't be played with easily. I tend to use other tools and then import it in.
Getting a layout right is all about fiddling.
I love tools like CSSEDIT, dabblet and jsfiddle that let you see your changes live.
Stylizer has some great panels that let you use sliders to quickly dynamically see how changing a CSS value like width would affect your design by using a slider to dynamically view the change. If you can see exactly what changes to your "code" are doing then you can learn quickly and easily.
WYSIWIG is a great environment for learning...
Adding live CSS editing tools to all the standard browsers is probably doing more to advance the quick learning of CSS/HTML5 than any other initiative. If you can play with lots of different examples live and immediately see the effect ... not only do you learn what it does ... you want to do more ... it is fun!
I would love dabblet or CSSedit with sliders (dablet already has visual explanations (e.g. a ruler graphic showing the width) of each manipulable CSS value but I would love to be able to interact with them dynamically) and quick help e.g. this is a list of possible values for this code element. To make it a perfect HTML5/CSS learning tool.
But I digress, so to answer your point.
No please don't restrict your tool to just allowing your users to input the text and images separately. Let them learn and grow in a step wise manner. Trust your users and give them access to both a WYSIWIG view for fiddling and creativity and the structure/code for developing an understanding of the underlying model. Oh yes and put sliders like Bret Victor's EVERYWHERE!