I think because of the Facebook integration allowing you to earn lots of coins via goals and score wars as well as cred being useless after you've unlocked all powerups and the achievements (1000 and 10000 cred). The whole system becomes pointless but the bigger issue I have is the road rage powerup. I think there should be some kind of way to earn road rage as it's essentially Blitz's equivalent to MW2's Tactical Nuke. For MW2, you have to earn a 25 killstreak which is a challenge. Regarding Blitz, all you need is to unlock it by getting enough cred and spend 300 coins. Not exactly a challenge!
Maybe a better option would be to increase the price of road rage to like 5000 coins or if you use it, it becomes unavailable for the next 20/25 songs (of which you have to play unique songs to get it back).