I can see where you get this idea from but your ignoring the fact that the Wizard might get a little in over his head when using spells that he or she cant handle. I see the wizard as an arrogant type of character, hence believe that the instability resource system will allow you to mash spells until becoming unstable and then having to do something else to bring your character back to stability.
I was thinking more along the lines of what the Wizard is destabilizing, not what is causing the Wizard to become unstable. From the quote I cited, I was imagining the resource coming from the Wizard corrupting the reality around themselves.
Yeah, that would also make sense too
Therefore by the Wizard altering reality and corrupting; power builds more and more. This makes the most logical sense to me.