Well quite clearly this is getting rather heated so let me introduce a few notes from my own experience using DIY C-Stands and before anyone goes tearing into me for being irresponsible, I come from an engineering background, I know how to build these things safely (paperwork to prove it! ;)).
So clearly there is concern about the materials used to make DIY stands. I have to agree, I would be a little concerned if someone used aluminium piping with $3 threads to hold a 5K but I don't see why you couldn't use those cheap materials to support something light like a flag etc.
From a technical point of view, the joints are the weakest point, I prefer to get steel threads as opposed to the brass alloys that you commonly see in DIY stores, steel is stronger and less likely to snap.
Another thing I like to do is put a little weight at the bottom of the stand (I use a heavier center pole for the lower segment), really all this does is lower the center of gravity and make it unlikely that should some incompetent crew member set the thing up ready for a light, then run off to find a sandbag, leaving the stand alone and dangerously close to a man who has a passion for bumping into things, I can be confident that it would take one almighty shove to cause the thing to fall over and potentially injure someone. Although this risk is present with professionally made C-stands too.
In most cases, common sense prevails but I'm always paranoid about these things, it comes from seeing the damage badly made DIY rigs can do.