Perfect Justice, or at least as much of it as possible, is critically important. Like everything else, the amount of justice we are achieving is improving exponentially. Any time you give up justice, or fail to include it in your goals, things quickly break down in many necessarily evil ways that reward the receiver of the sacrifice of justice while hurting the giver of such, and thereby necessarily, also visa verse. We have faith that perfect justice will be possible some day, and will never give up our hope for such and willingness to forever seek for such, till it is perfectly achieved, and known as such, by and for all. Whether in this life or the next.
We value distinguishing between two different types of "Mercy". There is primitive or lesser mercy, which could be defined as the sacrifice of justice. There is also a possible "perfect mercy". Perfect mercy is compatible with justice while primitive mercy is not. Primitive Mercy is laudable, at least in a temporary way, and only to the degree perfect justice isn't yet economical.
A good example of primitive mercy is illustrated in the parable of the laborers given by Jesus, as accounted in Mathew Chapter 20. Obviously, this story sounds strange to anyone today, as nobody would tolerate this kind of injustice and go work for anyone, with just a promise of go work for me and "whatsoever is right, that shall ye receive". Anyone would expect to know, up front, how much they would get, either on an hourly bases, or some other measurement of performance, before they would be willing to do any such work.
Obviously, 2000 years ago, there was little if any time or easy method of record keeping, and money tallying was also very crude. The smallest unit of money, the penny, was equivalent to an entire day of labor, at least for some. So even if the vineyard owner wanted to pay the others less, he had no ability to economically do so given the crude technology of the day.
We are obviously far more just today, but we are still far from perfect. I similar moral story today, could be something like the free software foundation morally railing against any musicians attempting to lock down their music or any kind of software, and not share it, unless they get a good reward. To be perfectly just, today, we should probably pay for something like .001 cents, every time we enjoy a song. While we're on the verge of having the kind of technology that would make such justice easily economically achievable, most people would still primitively think it absurd to try to track fractions of a penny for something like every time anyone listens to a song. Giving music away could be considered a type of primitive mercy, since it obviously isn't perfectly just. So, temporarily, it may be morally good, in a primitive kind of lower law way, however, we should never sacrifice our hope for perfect justice.
Obviously, any such type of primitive mercy is not perfectly compatible with justice. In the case of the vineyard laborers, the owner and society, in general, is in far greater debt to the people that worked all day in the vineyard than they are to the people that only worked for the last hour, yet received the same reward.
In contrast to this kind of primitive mercy, which clearly isn't compatible with justice; there is a different kind of perfect mercy which is compatible. If someone provides to a sinner, the ability to make a full restitution for their sins or debts, that can be considered as a type of mercy that is perfectly compatible with justice. This kind of perfect mercy is morally superior to simply unjustly 'forgiving' or 'forgetting' debts both of which make perfect justice impossible.
Perfect atonement and mercy is always only possible when everyone knows and accepts that perfect justice has been recognized by and achieved for all.