Coming from a competitive aspect, Poison has it's niche's and is actually quite a useful type for Pokémon.
Poison, most notably, is arguably the best type to handle Fighting types (fairy might be now, but we'll see). Fighting types come equipped with Dark, Ghost, Rock, or Ice quite often, negating Ghost, Psychic, and Flying's ability to handle them. Recently, many Fighting types come equipped with Poison to handle Fairies. Very few Fighting types will opt for Psychic or Ground moves, because they lack any notable type advantages bar Poison. A prime example of this is Weezing, who is near impossible to break with most fighting type that lacks Guts or isn't Medicham or Gallade. Poison also has some nice resistances against Bug (and to a lesser extent, Grass and Poison itself).
Poison is also a fantastic type to work with defensively. It's 2 weakness, Psychic and Ground, not only have immunities for each type, but are incredibly easy to prepaid for on a defensive team. Psychic types greatly struggle breaking anything with solid special bulk, while few ground types can hope to break Skarmory.
Poison is also famous on Bulky Offense teams. As these teams cannot afford to waste a slot on a spinner, the often run a Poison type to remove Toxic Spikes. A famous example of this is Skuntank in NU, who while is not very good at a lot of things, is specifically put on bulky offensive teams to remove Toxic Spikes and remove Ghosts/Psychics. In addition, many poison types can use Toxic Spikes themselves.
Poison also got a recent buff against Fairy types. While Steel may be a better answer to Fairy, Poison is significantly under-prepared for. Because so many pokemon learn Sludge Bomb or Poison Jab, you might find many of the Dragons running this move for the sake of hitting Fairy's hard, along side many Fighting types as well.
Poison is not the best of types however. It still lacks notable coverage offensively, and there are far too many things that resist it. But the most detrimental thing is that there are very few poison types that can heal. As it is a defensive type, Poison needs a good, reliable poison type that has solid bulk, decent offensives, and a reliable recovery move in order to survive for long.
Those are my opinion on poison anyhow.