i certainly see some value in it as a response. the problem is that it's force as a response is going to be dependent on just what the person you're arguing with believes about how we are saved.
to see what i mean notice that the response would be worthless against a Christian who believes that in order to be saved it takes faith AND works. and notice, too, that the response would have little force against a Christian who, although they believed that in repenting and asking Christ into your life you are saved, you are not once saved always saved, that you must continue to live a Christ-like life.
and it would also seem to have little effect on the liberal Christian who doesn't even necessarily believe that a person must accept Christ to be saved but who still believes that morality is grounded in god.
so, it has it's place in certain limited contexts but there are better more generally applicable responses i think.
if you had it in mind already that it was geared to that then well done. i think it certainly undermines that position.
though you might find that people that believe in that brand of salvation might also tell you that their motivation is because they feel a responsibility to god because he gave them life.
and it's this feeling of responsibility that motivates them.
My most common response to this is that the only morality recognizable in a modern society can only be achieved by ignoring the commandments of the bible. Yes there are moral commandments; but there are also abhorrently immoral commandments. You can even make an argument on the bible's illegitimacy as scripture on these grounds, simply by pointing out that if these commandments are the actual commandments of god, then they are in error following the other (contradictory) commandments- and that if, as they seem to realize, these commandments have very little bearing on anything and are not commandments to follow, that this is an error in the bible, which means the bible is not infalliable, which means it cannot be the word of god, and again, there is no compulsion to follow it or indeed to believe any other baseless claim it may make.