Nonviolence isn't Slower
There's a myth that nonviolence is slower than violence (even Gandhi believed this), but there isn't any evidence of that in history.
The nonviolent overthrow of Marcos in the Philippines, measured from the assassination of Benigno Aquino, took only three years. Mao Ze Dong and his Communist Forces were in Combat for 22 years. The Vietnam war lasted for 35 years.
Violence just feels quicker because we're dodging bullets. People are typically impatient, they try nonviolence for a week, then when it doesn't work they go back to violence, which hasn't worked for centuries.
Nonviolence usually succeeds twice as often as violence. Peaceful protests attract 11 times more participants than an average armed struggle. Nonviolence is also more effective in drawing participation from diverse segments of society. It pulls away support from existing dictatorial leaderships, allowing security forces, economic elites and civilian bureaucrats to shift their loyalties.
Naturally, some nonviolent movements fail, but there's no evidence that violence would perform any better.
Hard Enemies?
There's an argument that nonviolence can only work for easy enemies like the British, and can't ever work for brutal ones like the Soviets or Nazis. Yet if you look even at those cases, nonviolence has seen to be successful in all of them.
In 1968, Czechoslovakian civilians nonviolently restricted Soviet armed forces from subjugating their country. And during World War II, Norway nonviolently and successfully resisted Nazi attempts to reorganize its society along fascist lines.
Negative Side Effects of Violence
Violence brings about much destruction to property and the environment. There is some destruction in nonviolence, like during Gandhi's movement, hundreds of Indians were killed by the British. However that pales in comparison to the tens/hundreds of thousands, or even millions killed in violent revolutions.
Violence almost always ends up in repressive dictatorships. Because once rebel troops gain control, they keep acting as they're used to, i.e. they start running the country like a military camp.
In a violent struggle, one goads the other to greater aggression, because each side uses violence to justify its own brutality.
Plus after fighting, people will leave as long-standing enemies. The most amazing thing about Gandhi's nonviolent revolution is not that the British left, but that they left as friends (where Britain and India became partners in the British Commonwealth)
Source:
The Effectiveness of Gandhi's Nonviolent Movement:
http://www.markshep.com/peace/Myths.html
Scientific Research showing the Effectiveness of Nonviolence vs Violence: