Political decisions aside, planetary defense (not just detection, but intercept capability) seems like a great place to spend money and develop new space technologies with multiple applications. Even if we're never forced to use it, it will be perceived as good insurance, and the spinoffs will leave us with a viable space infrastructure which will also be useful for manned science, colonization, exploration and industry. The occasional training missions can also be planned to yield collateral science or development results (we can thoroughly survey a NEO before we nudge it or blow it up).

Viable but obsolete technologies can be sold off at a loss to private enterprise for further development rather than just being allowed to fade away, giving the private sector an opportunity to get a foot in the door and perform their magic. It seems to me like a great way to make the manned space program appear "useful" to even its most strident critics, and to internationalize the process and build up a constituency by providing contract opportunities to every nation and company with a space capability. The effort could also be designed to take advantage of existing space infrastructure, keeping worthwhile programs alive rather than allowing them to whither away from lack of interest. Planetary defense could very well be the high payoff item that makes space investment attractive. It would certainly trump finding commercially valuable life forms on Europa, or an abandoned alien base in the asteroid belt.

Sure, there are opportunities for pork, but there are with any big expensive project, aren't there? Pork appears to be just the price of doing business.