Yes, the DMX512-1990 standard requires you to use a 5 pin connector and 5 conductor cable. There was talk about being able to expand the protocol for other uses that hasn't happened yet. Manufacturers realized that since only 3 of the 5 pins were being used they could save money by using 3 pin connectors and 3 pin cable. In many new devices you have both 3 and 5 pin connectors, so some manufacturers decided they would spend extra money, but the end user pay it anyway, so who cares.
Is
DMX due for an overhaul? Probably not. There is
ACN which is starting to hit the market, but it is going to take a very long time before DMX goes away. DMX does just about everything we need it to do, and it has been installed in so many venues that upgrading will take a very long time. There are still people using analog dimmers that require Cinch Jones connectors, and they just hook their new console up through an A/D converter.
The reason that DMX is not used for triggering scenery or Pyro cues is due to the protocol itself, and has absolutely nothing to do with the physical properties of the cable,
etc. DMX is based on the RS-485 data protocol. The controllers sends out a repeating stream of information, consisting of a start code, and then 512 data blocks which are the levels for the 512 DMX "channels" that can be connected to one universe. Each device in the universe counts the blocks as they go by, and if the devices starting address is 100, it listens to block 100 through whatever and does what it is supposed to do. DMX has no error checking though, so if there is a bad block and it was connected to a pyro device, it could accidentally trigger the device. This is why protocols like
MIDI are used for show control.