Here's an example of a move becoming dual type while changing the metagame:
Extreme Speed : Electric/Normal (you choose the type upon being taught said move)
What does this do to the metagame?:
You've now just given quite a few Pokemon access to shut down Rainy Day teams. Which are considered the MOST POWERFUL and overly used team in general. (especially of weather types), You've now given a way for SD/Sandstorm teams to at least fight back for the most part.
Edit: Oh and an example of a Pokemon in competitive with a poor movepool which could move up a tier or tiers if it gained something:
The only sets run, run Ice Beam and Hidden Power.... Great coverage.... >.> Sarcasm.
Only insecure and ignorant people try to debase someone online based on their username.