I am not sure is MDI design patented but some people hate it
Its essential for CAD and other engineering software where you need to have multiple views of different aspects of the same object so you can see how adjustments you make in one thing affect others.
You won't find many big engineering software that doesn't use MDI.
The one one problem with my approach is that the canvases of the objects that make up the control are not actual forms, they are simulated forms.
Generally, when you move a form across the screen, it caches the bitmaps of the objects under the form so that it doesn't force the objects underneath to repaint. Without this cahing, the moved form would leave trails across the screen (which eventually clean themselves up but it looks unprofessional).
When you move a non-form object across the screen, the OS keeps sending paint messages to the underlying objects and everything looks messy.
I bypass this in my applications by putting just one frame on each tabsheet which contains my own designed objects (Like charts and schematics etc). These objects have a paint method that draws into a bitmap instead of direct to the screen, the bitmap is then copied to the screen.
This caching into the bitmap means that the objects only have to redraw when the user actually changes something in the object or the object size changes by the window being resized. For all other OS paint messages, the object just does a copy of the predrawn bitmap, thus the window dragging becomes smooth.
It is possible to create TMDtabsheets from the designer and drop controls on them as per any other container control, but if they don't implement internal paint caching, the form won't be as smooth when its dragged across the screen.