There is a temptation for Catholics—also present for non-Catholic Christians but with different terminology—to settle for avoiding mortal sin. After all, if one has living faith and does not reject Christ's salvific grace through mortal sin, one will be saved. So why should one worry about venial sin?
Leaving aside the question of purgatory—for that is not the heart of the issue, but something more in the way of an effect of it—here is one thing that is wrong with this. In a state of mortal sin, one is bereft of living faith, of charity and of Christian hope. One is spiritually dead. If one is not a state of mortal sin, then one is spiritually alive. But surely we are not merely satisfied with being alive.
It would be silly to say: "I shall not go to the doctor. Yes, I have a great big ulcer, but after all, I am alive, and that is all that matters." While there may be contexts where it is appropriate to shout with joy "I am alive!" as if that was all that mattered—for instance, right after one's life (spiritual or physical) has just been saved. But as regards the body, we do not just want life. We want a life of health. One can be alive, but very ill, close to death. There is still reason to have the joy of life—there is a qualitative difference between that life and death—but that is not what we aspire to. (Here, of course, one recalls what Socrates says about how happiness is thought to require health of body and in fact requires health of soul.)
But there is a disanalogy between the physical illness of those who are physically alive and the spiritual illness of those who are spiritually alive. For while this particular physical illness may not win out, our mortal body is after all heading for the grave—perhaps unless the eschaton intervenes.[note 1] But while spiritually ill though in a state of grace, there is reason to hope—not just hope to overcome that particular illness, but to overcome them all, by the grace of Christ living in us. Thus we do have more reason to rejoice over being spiritually alive than over physically alive—but this rejoicing cannot lead to idleness, since after all, how much do we want to prolong our ill health?