Morals, or values, are like projections of our inherent and learned preferences onto a more abstract and universal plane. We take our very subjective past experiences and the emotion that accompanies them, and reflect upon them (often there is a natural transition rather than a conscious effort) so that we may generalize them into a system of 'right' and 'wrong'. This system has at its foundation a well of emotion, whose relative depths determine the degree of conviction with which we adhere to and spread said values.
The topic to which you alluded in your original post is the conflict between these values and other desires. That is, what if we wish to partake in certain activities which are in conflict with our moral system? To this question, I would venture to guess that the degree to which such a system hinders practicality is the extent that such a system is impervious to change. What is practical is subjective, and dependent upon one's desires. To say that a moral system is impractical is to say that such a system does not adequately account for certain desires, and thus causes an individual to act in a way that is not in his own best interest (self interest, in this case, encompassing the entirety of his desires).
**gah, I'm done for now with this. I usually write more completely, but I'm having trouble expressing my thoughts today. I'll just let what I have so far be a springboard to others.**