There are really at least two issues here: 1) whether or not visiting a comet is a worthwhile mission, and 2) whether or not this is a practical spacecraft to do so, or just a kluge project to find a use for the Orion capsule (which is in danger of being a spaceship without a launch platform.) The following attempts to summarize the salient points of the above commentators, plus add my own spin; the second part is more entertaining.
1) Since my big dream is to have space travel become relatively affordable for "normal" people, preferably within the next 40 or so years I hope to live, I'm all in favor of this as a mission. It may be the smartest thing NASA has proposed in years, and the single most important factor in making space accessible to the rest of us. Depending on who's figures you choose to use, it costs $5000 to $15000 get a pound of mass into low earth orbit; for practical space-faring considerations, any long-term lunar or interplanetary mission fiscally implodes based on the amount of hydrogen, oxygen (for air, water and rocket fuel) and other supplies needed, if you are lifting them from the surface of the Earth. Prospecting for those resources on the moon raises other problems: A) they haven't found large quantities of water ice on the moon yet; B) the oxygen and hydrogen that they have found will require a lot of energy to process a large amount of ore, which is slow and expensive; and C) then after you process the stuff, you have to use more energy to get the usable products off the surface of the moon. Near Earth comets are the cheap, elegant solution, as they're already in high orbit and they're already mostly water, so they'd be really simple to process. I think it may turn out that a permanent moon-base will be better served by landing cometary resources, rather trying to prospect and process locally. Similarly, cometary resources would enable a much larger space station, with manufacturing and processing facilities of its own, possibly hydroponic farms, and maybe even the dream of a space hotel, where you could take a real shower.
2) So while I'm totally sold on going to comets and learning how to exploit them, I'm not sure the twinned Orion is the way to do it. Does this even need to be a manned mission? It would be faster and cheaper, (especially since it looks like Ares 1 is going to be delayed, if they can even get it to work), to send a robot prospector, which in concept might be the lowest tech space ship ever flown. A mosquito-like robot would land on the comet, insert a water heating probe, suck off the gasses and liquids, and then inflate a large bladder using a blast of steam. The bladder would be allowed to freeze, creating a giant tank, which would then be filled as the water heater keeps running. You would end up with a kevlar bag around an ice cube. The robot would then use some of the water as fuel to get back to the International Space Station. A variation would be to catch the precipitate as it comes off the comet, which would be more complex, but less likely to destabilize the comet's orbit. Regardless, as a test project, if the robot only delivers about a ton of ice, which would be about a 264 gallon volume by my calculations, it would be worth between $10 to $30 million. Potentially, this could be the first spacecraft--who's primary function isn't to process data--to ever directly pay for itself; especially since the robot prospector would be reusable, and after the first outbound mission, provide its own fuel. A Ron Popeil flash, "wait, there's more": If you filled a kevlar bag with snow and/or gas, and attached a propulsion unit, (basically, a retired robot prospector), it could serve as a cheap catcher's mitt/airbag for all that space trash, which is becoming more dangerous. “And if you order now”, you could make a robot sled to retrieve satellites that need repair and upgrades, and return them to high orbit, using the comparatively cheap cometary water as fuel. (Given the cost of building and launching satellites, I’m surprised that they haven’t developed this capability already). Here's a new dream: maybe someday NASA will reward ideas with seats on spaceships.
All of this comes down getting to the comets as soon and as efficiently as possible.