If it's to be integrated in the emulator it is not a "new plugin" feature, it need to be integrated in the GS plugins, and nowadays the main task is getting things running smooth yet.
For now it is an option for those that have enough horse power to run it as it is. It need to be clear, it's not free of cost, each shader program is indeed at least the vertex program, mostly 2 programs, the vertex and the fragment programs, this last acts in each and every 'to be pixel' in the vertexes... let the imagination work and notice this can't be a free lunch.
Besides, since the PS2 itself is not that Graphic wonder, many newer video card indeed have the spare horse power to deal with those shaders and so it can be an useful feature (at least for those with really powerful machines).
PS: those mods and plugins are really wonderful, anyone that knows what was done for the now ancient and marvelous Infinity Engine games like Baldur's Gate and Planescape Torment to name only a few, the Morrowind itself and even Oblivion got it's share in graphics enhancement knows. And I agree, the modding community is amazing as their mods themselves are. But we must not lose the perspectives and priorities.
PS2: It helps the mod acts after all the emulation work was already done, it will be almost the same weight does not really matters that much the actual game being played. It still depends on what remained in machine power, if it can run in a 'free' core... hmmm, I'm attempted to see it action
Notice the mod is not "under" the program it drives, it's over it.