This question is really a philosophical one.  It is rooted in logic and is attempting to make sense out of the relationship between effective prayer and justice/fairness in the lives of those for whom we pray.  I do not believe that philosophy or logic can provide a sound basis to answer this question.  However, as a person of faith, I do believe that a biblical/theological response is fully adequate.

The following are a few points based in scripture that provide a sound basis for answering the question:

1.  Intercession is often associated with prayer, but prayer is only one form of intercession.  Merriam Webster's Collegiate Dictionary (Tenth Edition) defines intercede as "to intervene between parties with a view to reconciling differences:  MEDIATE."  Jesus interceded for all of fallen humanity when he died on the cross for us.  The Bible says, "For he bore the sin of many, and made intercession for the transgressors."  (See Isaiah 53:12).  Now, if Jesus had not "made intercession" for us, we would be eternally lost and cut off from God.  

Where does justice and fairness fit into this picture?  If God gave us what we deserve, we would be eternally separated from Him.  That would be fair.  But, because of the intercession of Jesus, we are forgiven our sins and reconciled to God.  That's grace.  

There is an insight here into biblical prayer.  Intercession is a means by which to stand between God and another person and appeal to God to withhold what is just or fair and to give mercy and grace for Jesus' sake.

2.  As Christians, we are called to represent Jesus in the earth by partnering with him in the ministry of intercession.  We are referred to in 1 Peter 2:9 as "a royal priesthood."  Royal means we have regal authority since we reign in life through Christ.  Priesthood means we are intercessors who stand between God and others.  

The reason we call this website "Advocates In Prayer" is because we come to God on behalf of others as Intercessors/Advocates --  defense attorneys, if you will -- saying, "God, I know this person may not deserve your blessing.  None of us do.  But, I remind you that Jesus bore our punishment at the cross to forgive us, deliver us, reconcile us, and bring us into a life of favor and blessing.  So, for Jesus' sake (representing Jesus), I ask for your mercy for this person, and I pray that you will meet their need for your names' sake, to uphold your honor."  

Incidentally, an attorney calls the judge "Your Honor."  When we ask God to meet someone's need based on the atoning work of Christ for that person, we are appealing to uphold God's honor in that person's life.  Our prayers turn the judgment seat into a mercy seat.  Is this fair?  No, what's fair is that everyone get what they deserve.  Prayer appeals for God's mercy, not for justice.  Yet, because of what Jesus did in taking our punishment, God is "just and the one who justifies those who have faith in Jesus."  (See Romans 3:26.)

3.  The Bible teaches that if God cannot find a person to intercede for others -- calling upon Him to be merciful to them -- then He has no choice but do deal justly with them and punish them for their sins.  Consider this passage from Ezekiel:  

"I looked for a man among them who would build up the wall and stand before me in the gap on behalf of the land so I would not have to destroy it, but I found none.  So I will pour out my wrath on them and consume them with my fiery anger, bringing down on their own heads all that they have done, declares the sovereign LORD."  

-- Ezekiel 22:30   

This is a clear example of where God wanted someone to pray for His people Israel, pleading with Him on behalf of His covenant to show them mercy.  However, since there was no intercessor, no advocate, no defense attorney to turn the judgment seat into a mercy seat, God had to bring judgment to give the people what they deserved.  If this seems to paint a bad picture of God, consider the fact that God's plan was to come Himself in the person of Jesus and pay the penalty for our sins so that He could remain just and still be merciful to us.

4.  Consider Exodus 17:8-16.  This passage speaks volumes about the relationship between intercession and the protection, blessing, and triumph of those receiving prayer.  When Joshua led the armies of Israel into the Valley of Rephidim to fight against the Amalekites, Moses went up on a mountain to intercede before God on behalf of Joshua and the armies of Israel.  When Moses' arms were extended to heaven in intercession, Joshua and Israel prevailed in the battle.  When Moses' arms got heavy and he let them down, the Amalekites prevailed.  So, Aaron and Hur went up the mountain and stood on opposite sides of Moses to help support his arms so that he held them up till evening.  Thus the Israelites won the battle.   

Joshua and his troops could not pray and fight at the same time.  They needed Moses to intercede for them.  This is a perfect Old Testament illustration of the high premium that God puts on intercession.  

Each of these insights and illustrations from scripture drive home a very important insight into prayer:  Much good will come to the lives of others because we pray that would not come to them if we did not pray.

Every Christian has a covenant relationship with God through Jesus Christ.  God will hear and answer their prayers when they are prayed in line with God's will and offered in simple childlike faith.  If a believer is negligent in his prayer life, God will often inspire and honor the prayers of others to make the way for Him to impart the necessary grace to minister to the needs of the one on the receiving end of prayer.   

There is a wonderful Christian group called Heirloom that has a remarkable song about this aspect of prayer.  The song is called "Prayer Warrior."  It's about a woman who worked a regular 9 to 5 job.  But, in her free time, she was a prayer warrior.  Consider these lines from the song:

"And we'll never know of all the good she's brought us;

We'll never know the evil we've been spared.

Many nights she's crashed through Satan's strongholds,

Reaching heaven with her prayers.

She's a prayer warrior, down on her knees,

Wresting with powers and principalities,

Standing in the gap for others,

For her sisters and her brothers,

Reaching heaven with her heart.

Prayer Warrior.

As this song implies, prayer is warfare.  In real warfare, your friend's life might be spared because you stood in the gap for him and warded off  the Enemy when your friend was weak, exhausted, or just plain negligent.  It's no different in spiritual warfare.  Jesus said "watch and pray."  We're to be alert when we pray in order to recognize and thwart demonic devices and/or strongholds that are set against another person.

Prayer is not about bringing justice, but about appealing for mercy.  We are Advocates In Prayer, and an Advocate is never a prosecuting attorney but always a defense attorney.  As we intercede for others based on the New Covenant sealed in the blood of Jesus Christ, our Intercessor/Advocate in heaven, our prayers turn the judgment seat into a mercy seat in behalf of those for whom we pray.  Our prayers are God's mercy extended.