as structural members
What the world needs is an adult construction set like they have for kids. You buy the blocks (or whatever) and make whatever you want. This does not exist except in the wood format. You can make pretty much whatever you want with wood ... but not metal.
The problems with Black or Galvanized Pipe
There are no problems with this medium ... if ... you want to transport liquid or gas. That's what it's designed to do. The problem is using it as a structural member for which it is not designed. It works (after a fashion) but just barely.
The threads of such pipes are "tapered". That is, they do not work like a threaded bolt. They are designed to snug up tight and seal. They are not designed to be used for ladder pegs on a telephone pole. Typically, you find about ten threads on the end of the pipe. In the connector there are usually about seven threads. But when you thread them together you use only about two threads. In other words, you get to turn about 720 degrees and the pipe has reached the limit of its travel. The rest of the threads are just manufacturing artifacts.
This is great for pipe-fitters. They just have to turn the thing enough to support its own weight and it's sealed (with a little goop on the threads). Imagine the swearing if they had to turn it ten times around for every connection in order to seal it.
This means that when you put torque on the pipe at the free end, it tends to strip out the two threads, i.e. there is tremendous leverage out there, say, 24" away on a 1/2" pipe (ID). If you could thread 7 threads, the resistance of the threads to being stripped out would approach the strength of the pipe against mid-pipe breakage which is structurally significant. I know from doing it that if you threat a metal pipe into a PVC connector (in this case you get about 3 to 3 1/2 full rotations) ... you can strip the threads out by arm strength alone using 18-24 inch pipes as levers.
With metal to metal connector, the strength is increased perhaps tenfold, yet it's still not anywhere near the actual breaking strength of the pipe itself. So, thumbs down on its use as an unbraced structural member, i.e. if you want to put great force on the pipe you must brace it with an angled piece. But you now run into a problem with connectors. There is not 45o angled "T" so as to allow you to construct a truss. You have to "rig" a brace with 90o T and 45o angle connector.
Virtually any attempt to use piping structurally results in a jury-rigged construction. It's just not designed with load-bearing in mind.
The problems with PVC piping
The same problems as before prevail here because PVC is meant to emulate metal, i.e. it is meant to carry liquid and perhaps gas in some situations and ... to seal rather than support weight. It bends and can be destroyed by vandals with hammers and cigarette lighters as I've seen in some playgrounds.
Still, people try to make structural things out of PVC. Where my mother-in-law is hospitalized, they make specialized wheel chairs and such for individual patients to get around in. It works, after a fashion in a venue it was not designed for. There just isn't anything else to choose from.
This material can't be threaded because it is too thin but has excellent connectors made from cast aluminum. Unfortunately, the connectors are not made with weight bearing in mind and the set screws would undoubtedly loosen and pull off if subjected to vibrations and random pushed and torques. But you can use a pipe bender on this medium ... so many more things are possible than simply carrying electrical wires. PVC (which also can't be threaded - but has molded threading) is also used for electrical conduit.
I tried to imagine using this stuff for my LCD wall mount because it looks "cool" but had to give it up as overly complicated in comparison to simple black pipe.
Articulating the structure ...
Here is a huge problem. Not only is all this stuff not designed for structure loads ... it's not even close to being capable of articulating an arm movement. If you want movement ... you're into the twilight zone of Home Depot. I asked the man there today "Do you have any bearings?" ... "no". So, you must invent a friction bearing like at a metal fence gate. Of course, you could always cannabalize some machine or whatnot ... but this invariably leads to hard decisions because nothing ever fits when you are "jury-rigging" a machine ;o)
I tried using the threads as articulating bearings. You screw each end into a fixed connector and rotate the pipe so that as it moves one way it tightens on one end and so loosens on the other. In general, this works ... but ... I had the misfortune of playing with the articulation so that it heated up as un-oiled metal will do. I let it get too hot and it froze up on me like a hot engine with no water in the radiator. When I forced apart the pipe and connector, some threads broke off ruining the pipe. So, this tactic is only marginally viable.
There is another type of articulation besides rotating arms and hinges. It is the sliding movement ... like an extension ladder. Here you run into ID-OD problems. The outside diameter of pipes generally coincides with the inside diameter of another pipe from the same maker. In other words, you can't insert the smaller pipe lengthwise into the larger pipe because there is not "slop" to allow that. If you get it in at all, it will seize up due to minor imperfections in the steel or plastic or just due to asymmetric thermal expansion or contraction. You need a 64th inch clearance to begin thinking of sliding pipes. The other option is to rig an "O-ring" to take up the open space in a looser pipe to pipe configuration ... or ... you can do what I did in one of my extension configurations. You can put two "T"s on the side of a pipe and run two other pipes through the Ts and slide the middle one, i.e. it's a jury-rigging but it works fairly well.
What I want to see
I would like to see a pipe set with nut and bolt type threading. I would like to see left and right threads on the ends of some finishing pipes so that I could complete a "loop" of pipe using the last pipe as a "turnbuckle" (the thing with two eye bolts in each end of a frame that you turn and which then draws in the two eye bolts thus tightening the structure.
My ideal pipe set can be used with a pipe bender and can be cut with a cheap pipe cutter. Though the foregoing is incompatible with threading, it would have EMT-type connectors for the ends ... but ... my connectors would have more threads ... they would thread like nuts on bolts ... and ... there would be a tool to hole the end of the pipe so that the set screw could go into the hole thus preventing it from coming loose.
Pipes would be sleeved with insertion in mind, i.e. you could put one smaller pipe inside of a larger one and have just enough slop to allow some movement in, out and rotation. Some simple (not too precise) bearings would be available (roller and ball) with ODs and IDs in mind. There would be hundreds of various connectors and a catalog to order odd pieces.