There're some outstanding after-market Festool inventions and accessories (the TSO guide rail square is one of them), but in my opinion, the domiplate is an unnecessary addition for an average domino joiner user.
For one thing, even if you owned all three different versions of the product, you could handle in theory only a limit of 6 different types of thicknesses in the way the accessory is designed for or to work with. But most importantly, it adds an unnecessary layer of confusion to operating the machine such as relearning how to use the machine as designed by Festool when you need to use it without the dominoplate -- say, doing a mid-panel butt joint.
The best way to master the domino joiner or any tool is to watch how people use it, use it yourself, do enough projects (and make enough mistakes along the way), and consolidate your experience. I do butt joints, mid-panel joints, offset joints, angled joints and even three-way mitre joints with the DF500 as is -- without using any after-market accessories. It shows how well the machine is designed (of course even perfect machines can be improved over time). The image shows part of a project being dry fit this week using dominoes and everything you need to make all those joints is covered in the manual.