Well the move to reduce emissions has to be global, it can't be unilateral if you want it to work.
I've heard this canard for years and it has alway struck me as illogical on several levels. Perhaps you can enlighten me (and others) on why this makes sense to you and other skeptics. Your statement distills down to "Why bother?"
From a top-down point of view there is a strong case to be made for mitigating an impending crisis as soon as it is recognized - this approach has several names: lead by example, take the moral high ground, a stitich in time saves nine, and so forth. It may be expensive to reduce GHG emissions and transition to more sustainable energy sources - but it is cheaper today than it will be in, say, forty years. Energy, materials, and labor costs are all rising. And, yes, I've studied the economic models that discount the cost of future mitigation efforts by claiming that we'll all be richer in the future. Those models are based on assumptions of unlimited resources and unrealistic economic growth - so they are about as plausible as predictions of flying cars and vacations on the Moon. If you have a broken pipe or leaky roof is it better to fix it now - or ignore it in hopes that you'll win the lottery in a month and be richer? (hint - it's smarter to fix it now)
Another advantage of tackling GHG emissions now is to make money. As many have pointed out, new technologies will be needed - and the countries that develop and market those technologies will be the ones to reap the profits. Shouldn't the US at least be in the running? If the answer is yes then we should be investing in R&D in the technology fields that will be needed and encouraging entrepreneurs to start up new ventures to implement the technologies. Granted, a large percentage of the startups will fail but that is true of any industry.
Your assertion fails from a bottom-up, grassroots, point of view, too. If your neighborhood is threatened by wildfire is it pointless to unilaterally save your house unless you can save all of the neighbors' houses, too? Or, since we are talking about the behavior of other countries perhaps a different analogy would be better. If your neighbors are engaged in an activity you know is harmful, say, dealing drugs - are you seriously saying that unless all of them stop their drug dealings it make sense to join them and start cooking meth in your basement? If you can't beat 'em then join 'em, right? Doesn't it make sense to do what you can - even knowing that your efforts may fall short of completely solving a problem?
I know that my reducing my GHG footprint is not going to make a noticable difference in AGW - but I also know that there are millions of people like me and our aggregate efforts are orders of magnitude more significant. Change - whether it's civil rights or recycling or reducing GHGs or whatever - begins on an individual level. Why bother? - because it's important.