I don't think you are missing anything with that question. None of that is really necessary, the way I see it, but some of those things might be necessary depending on the needs or desires of the person and his or her landscape; but I will have to clarify what I mean. I think you are right to question the over-complication of these things, and that the simpler the system is, the simpler it is to build, maintain, and generally deal with into the future.
I guess I just don't get why people are so keen on holding tanks, strainers, switches... why do all that?
What am I missing?
I have to respond: Maybe. But it might not be getting the best use out of it. Without a really good biological system already in place, a trench of this nature can create an anaerobic slime seal and back up on itself, or (in an area like mine) can get penetrated by a deep frost, stopping flow.
the secret of grey water use is getting it in to the ground as fast as you can, and letting nature do it's thing.
What is often done in situations/projects like you are describing is to have your exit pipe open up in your trench under something that amounts to a half barrel, cut from top to bottom. The open cut flat edges side faces down, and the half round side without edges faces up. Some people use plastic for this barrel. It lasts longer underground. They actually sell something designed for this purpose. The purpose of the barrel is twofold: To keep tree roots out (via air pruning) and to provide air. The best thing that you can do, beyond that is to get worms active in your trench. They will provide drainage and will consume microbes (both anaerobic and aerobic), creating humus which will allow most of the nutrients and the water to be held in the soil systems/be available to plants.
You do bring up a good concern though... something about an "anaerobic slime seal" which I don't know exactly what that is... but I can picture what it must be like. So I guess some thought must be given to preventing blockages... and in this case you are implicating a lack of air. I guess that is why I mentioned just passing it though mulch (just something I've heard being used a lot). For the most simple gravity flow situation... how does one plan to avoid such a blockage? I assume with providing for the right biology, yes?
Well, not simple, but without all the technology that you mentioned. Simple in that the pipe just drops it in, and simple that I am still direct as soon as possible to plants.
Well it sure sounds like you are doing all you can to simplify it!