IIRC, a majory criticism of the pipeline isn't so much "more oil is bad" but "this shit is unsafe, yo." Again, using memory since late night but the pipeline runs over domestic water supplies, and isn't really up to a great deal of safety standards. But it will be constructed anyway. Why? Because a government with the best interest of the public at heart is a lovely dream we all once had, and nothing else.
There are two major criticisms, I think: the compression factor, which would exponentially increase any disaster that occurred, and the "dirty oil" factor. I think both of these points are somewhat overblown, although both are valid. There's just not a lot of arguments to be made against stuff like this until we really, truly have an alternative.
I don't think its worth the risk. Remember that this is a relatively new and not well studied type of oil. If the risks were understood there would be much less criticism, but tar sands oil is not something we fully understand as a compound nor do we have the data to really analyze just how bad spill will be. Regular spills might be difficult to clean up, but they are understood. Its like having spider nest in your room. New species too, haven't heard things about it. Is it poisonous? Who knows, but it doesn't look friendly. Then someone comes in your room and says they are going to use a series of plastic hamster tubes to bring lots of the spiders to the other side of the room, because it needs spiders. What if the spider tube breaks? Wihat are the consequences? I'm not huge in to environmentalism, especially in the United States. I think much more needs to be done concerning forests in the third world, but just because my eye roams international doesn't mean we should be ignoring things at home. We certainly shouldn't overlook a large unknown like tar sands oil, and we shouldn't be pumping any of it anywhere without first understanding as much about it as we can. What are its effects on small environments? Large? Does it sink in sand;? What impact does it have on oxygen levels in the water? How efficient is it in terms of oil production? Will current safety standards be effectivve? Those questions and more need to be answered before construction should even be planned.