Your point is invalid. All you've done is state facts. XanderO even said to look past the typing and understand how some Pokemon have their own individual advantages, and you just jumped right back in on the typing - "Poison is weak to this, Poison is weak to that....blah blah blah".
I actually consider the Poison type one of the best in the games. As mentioned, the Poison type is a very, very frustrating type to be on the opposing side of. Moves like Toxic and Sludge carry a very high risk of Poisoning your Pokemon. With Toxic, the Poison worsens every turn, and Sludge is an offensive attack too. I've had many controller throwing moments at the hands of these moves, and others. I can guarantee that if we have a Poison type League Champ one day, he/she will be one of the hardest.
Poison types have their advantages and disadvantages just like any other type. It isn't all about type advantage. I can understand why people would think this though - I bet that the people who think Poison types are underpowered are the ones who use nothing but offensive attacks in their battles.
And if you find you're up against a Ground or Psychic type with your Poison type Pokemon - SWITCH OUT. Just like you would in any other situation.