I think it certainly would. We're not adding a weakness to whatever type, we're adding it to the most common type in the game. This would make Poison types useful, at least a little bit, for once. Also, it would nerf Water a bit, because it's overpowered.It still wouldn't achieve anything. Poison's offensive power is awful, so giving it another advantage won't help at all, and it definitely won't balance anything.Because a lot of the type match-ups are realistic and logical.Why should Poison be strong against Water? While it'd definitely be realistic and logical, when have Nintendo games had the need to be realistic and logical anyway?.
And because Poison is a weak type and Water is very strong, balancing both wouldn't hurt.