Often, protests and acts of civil disobedience are primarily designed to raise public attention about an important issue. The protests in Washington against the Keystone XL pipeline will do that. They could also have a substantial real-world effect if they succeed.
The Keystone XL pipeline would be a gigantic carbon highway, transferring carbon from the oil sands of Alberta where it is now safely buried to oil refineries, gas tanks, engines, and ultimately the atmosphere. There, it will cause warming that will persist for thousands of years. Keystone would perpetuate our fossil fuel addiction.
If Keystone is blocked, less carbon will be emitted between now and when the world finally comes to its senses and gets serious about climate change and moving beyond fossil fuels.
Keystone XL is also a waste. When the world does get serious about the need to stop adding carbon to the atmosphere, it will need to be shut down. That will be economically wasteful and politically difficult. It would be far wiser to invest now in improving energy efficiency and producing energy from safe zero carbon sources.
Hopefully, the protests starting tomorrow will press President Obama to take these arguments seriously and say no to the dirtiest and most dangerous oil in the world.