But will it matter? You have the chance to earn more or less in a week, but mostly its an average. Because it is more or less you wont lose anything in the long run. Of course you can earn less in the first week and more in the next week when the difficulty raised but it can be the opposite way too, so i dont see that this is a big problem.
It matters for someone. Consider your employer pays you either $200 or $300 this week depending on pure luck. Somebody would not take that risk(for ex. he has a loan to pay). Risk alleviating instruments cost real money.