DrQuint
You are also accurate about one-pay vs. repeat-buy powerups, it's largely a matter of cost and player viewed value. Things like fireworks, however, have an unspoken charm to them related to holidays, making it far easier to resell them every year. And let's not forget.... Facebook. I won't comment further on that.
If the powerup does need to be limited, then a daily timer should be used for it, not a limited uses model. On this, we agree fully.