Impossible possibilities and other what-not
That said, the frustration with mortal existence that lies beneath the ontological argument speaks powerfully to Mormon experience. We are terribly aware of the fallen status of the world and, specifically, of ourselves. In Primary and Seminary we are told over and over again that we are all sinners, that there was only one sinless human, and that human was Christ. The need for an infinite atonement is plain when one considers the nearly infinite amount of sin that humans have engaged in. Add to that the failings of mortal bodies, the impermanence of mortal substance, the fleeting nature of our lives, homes, memories, and natures and we can understand the longing for perfection that led the medieval Catholics to make the ontological argument.
At the heart of the ontological argument (and most arguments about the nature of perfection) is the desire for stability, for order. Why is a perfect God unchanging (the same yesterday, today, and forever)? Why is God's course one eternal round? Why are all concepts of God caught up in this idea of eternity? It raises God above time and the reality of time, more than anything else, keeps mortality from perfection. The awareness of the passage of time reminds us of our own mortality. Our mortality causes us to divide up and parcel out our time to those projects that interest us most. If we had infinite time at our disposal, we would never have to choose (or, at least, make hard choices) because there would always be time to do whatever you wanted to put off. But such is not the case. We have limited time (the exact limit, no human really knows) and we even spend a third of it asleep. God's perfection may even be the result of infinite time. But his eternal status keeps him firmly out of our understanding.
So, when we discuss perfection, we look at what we think perfection might be and we tie it into infinity. If I had infinite time, I could learn everything that there is to know; I could exercise and eat right until my body was perfect; I could create an infinite number of children and use the infinite knowledge gained by infinite study to raise them perfectly. Obviously, there is more to God than infinity, but what that would be is, by definition, beyond our comprehension, so why bother with it?
Here is the thing. We are not sent to earth to become perfect people (at least, not in the way Christ was). We don't have enough time. We are too willful, too flighty, too mortal, and too sleepy to really do it in the time alloted to us. Occasionally, we may have good moments, but, as President Hinckley has said, life is mostly smoke and bumps. We don't have a good idea of what we are about most of the time. We don't even have a good definition of what perfection is. Any definition is necessarily limited by our own experience which is another reason why the ontological argument doesn't work; recognizing that we are less than some arbitrary standard does not mean that we have gotten the standard right, all it means is that we are not as good as we think we ought to be. This makes self-satisfaction the equivalent of perfection, which I am fairly certain misses the point.
Now I could go on about the nature of grace and the combination of faith and works, but others have done it better and it's probably a subject for another day in any case. My point is this, the world is not designed to produce perfect people. Time and mortality in all things work against this and they are excellent foils. We are all damned to imperfection, no matter what. So, why ask us for perfection? That's the paradox at the heart of our interaction with God. Only by asking us for perfection can God cause us to realize how inadequate we are to the task. Having realized that, we can turn to Him who perfects us via His grace. Thus the commandment is kept when we realize that we can't keep it without God.
So, whatever perfection is (I again assert that we don't have a helpful definition), we know that we aren't it and that God is. So it is only via union with God that it is achievable. This union is only possible through the mediation of Christ and his atonement, in other words, it is there for the asking.
Why should we not care that we fail to be perfect? Because it is an impossible task. It's like beating yourself up for failing to fly by merely flapping your arms. We need help to soar.
ps. After writing all of this, I realized that everything I wanted to say had been said in the comments to Carrie Ann's post. So, happy repitition to you all.