If you have strident political opinions and you want to make change happen, music might not be the best medium to address your cause. Yes, there are scattered examples in hip-hop history that show music's political potential, its ability to convey powerful messages, with Public Enemy being the most obvious and most successful. But these are rare.
An artist's ability to create meaningful change is also dependent on being heard by an audience that might act upon what they've learned. A lot of times, a political message gains power from being personal; rather than hectoring. Some of the best rappers would lead by example. (Think 2Pac's "Keep Your Head Up," rather than the condescending "Brenda's Got a Baby.")
This isn't an argument that music can or should disregard moral purpose; quite the opposite. Instead, artists should simply recognize that by labeling oneself as a "conscious" or "political" artist, a rapper is setting up himself or herself to merely preach to the choir. All too often, their decision to be "political" is more about branding than it is about creating real positive change.