there are many more uses for tess in RT rendering than just displacement mapping, btw.
shadows are usually not very precise anyway, so semi-microscopic details like those that would be introduced by things like displacement mapping are not generally expected to be accurately reflected by SM. Heck, in most engines you can't even expect it to accurately reflect all objects in the scene at all.
I'm not sure I follow your baked lighting example. Tesselation is generally well-behaved with baked lighting. If tesselation causes clipping in your objects or other topological changes, you messed up anyway. But that said, there is nothing that really prevents you from baking at a high tess level, if you insist on having bogus geom/tesselation. Bottom line, if getting shadowing errors from tesselation is a common occurrence for you, either you or your engine must be doing something wrong.
Either way, there are many ways real-time rendering can go wrong (it's all just a big hack, after all), not having tesselation/displacement mapping that is so crazy that it creates artefacts is just another one of the many things that you have to look out for.
There are other ways to use tesselation than displacement mapping, though. It can be used for all kinds of things -- smoothing out objects, UI elements, reducing GPU transfer buffer sizes, [procedural] water (that's a popular one), cloth, ... etc.
As example for cloth, your physics engine might compute a lattice of 100 x 100 nodes, that change every frame (since, well, physics.) 100 x 100 looks too crude to present to the user, but the physics engine can't deal with more. So now you could either
- on the CPU, tesselate the mesh of 100x100 quads into, say, 500x500 quads, triangulate them, then push the whole buffer (which is now 25x bigger, but #YOLO) through the PCIe bus to the GPU where the GPU can render it
- just push the 100x100 buffer to the GPU where it gets tesselated and triangulated by the hardware in no time