There are about 300 spills a year from pipelines. You can do the googling yourself, but that's how many spills were considered 'significant' in the USA in 2014.
I tried to find a source that isn't obviously left-wing so you wouldn't reject it out of hand: http://www.forbes.com/sites/ja...
Admittedly, every single method of transporting oil is bad, and all of them lead to environmental problems and costs. So while pipelines may be 'best'--for some meagre definition of 'best'--there is an absolute amount of damage that's done. Conventional bombs are 'safer' if you drop them on my house than nuclear bombs, but it's a distinction without a difference when it comes to whether or not my house is still there.
Maybe the maintenance is as good as we could hope (I doubt it; we're talking about fallible humans and corporations with a bottom line), but this isn't exactly a solved engineering problem.
Nobody really knows how bad a bitumen spill would be. There's a lot of extra chemicals in there to keep it diluted and flowing.
In the end, one catastrophe a year is too many, even if it seems like 20 years later maybe things would be okay. I'm dubious about that number; 25 years after the Exxon Valdez spill, Prince Edward Sound is *mostly* recovered but still has problems. 20-30 years is a big chunk of time to humans, and we still have to live in this world.