I guess you answered yourself right there... Games can be decided before turn 5. Not the "kill the opponent" kind, but if they got a Puwen and an Aldon out, your turn 5 Plasma won't do much good. If you have an answer (say Lightning Strike), you'd probably kill the Puwen, and they'll play another ally if they predict that you have Plasma. Depending on your draw, you may never actually get a chance to cast it.
If it slips into play, though, you still have a lot of ways to deal with it. Any sweeper, Mind Control and Retreat just to name a few.
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