mirrorx mode is especially set to be used with rendertarget to transform in real time the camera view around x axis (imagine a camera rotation along x axis). There is no real interest to be used in a live camera because it only translate y pos and target pos of the camera (you can do the same with a live camera). you can use this mode with modmat et simple mat like others rendertarget, your view will be transposed (see example). Maybe "Mirrorx" name is not really a good choice, because it does not mirror anything, just prepare view to be mirrored.
The UpdateLastView allow to update a camera information (matrix transformation) that can be retrieved later by others cameras. (for now only mirrorx mode use it and free camera mode when you press 'o' key), but it can be usefull for others mode later
. In case f waterreflaction, it allow to update the camera in mode mirrorX with the same position than last last camera and it apply after its -y transposition. (try in example to desactivate it when rendering scene from last camera, howver take care to not click in the 3d view, because it also update this information, if you doesn't activate it in waterreflection example, reflection will not be correct because the camera position in MirrorX will not be updated correctly).
So mirrorx is more especially used to do reflection along a plane in conjunction with a shader that can manage it. like the initial example of water reflection shader.
To do other type of reflections (reflection arround a mesh) we can use env mapping with envi-reflect/sphere in tex2d uv mode, even if it is faked it can be it can be good enough used with render target or skyenv. Else, to be able to do a real time reflection around non planer mesh it need a raytracing technique, it is another story
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