Luke 18:1-8 is not an exact comparison of God to this unjust judge, teaching us that we should badger God until we weary Him and He grants us our request.

Jesus is contrasting His willingness to answer our prayers with this unjust judge's unwillingness. The parable is a contrast, not a representation.

Not only do we have God who is a just judge who will avenge His elect speedily, but we also have Jesus as our advocate or attorney who is always making intercession for us. 

We have an adversary (the devil) who is constantly accusing us and misrepresenting God (the judge). This causes men to give up (faint) and not even plead their case with God because they doubt that He will answer them anyway.

Jesus is saying that our Father is not an unjust judge that we have to pressure into doing what is right. Many times we put more faith in people and their willingness to do what is right than we do in God. 

Satan has deceived us about the willingness of God to answer our prayers and Jesus is countering that deception with this parable. Jesus is encouraging us to pray (petition God) and not doubt His willingness to grant our requests. To teach that we must pester God until He gives in to our pressure is not good theology.

This widow's actions were commendable. She knew what was rightfully hers and she refused to take "no" for an answer. If we can be that confident and determined when dealing with unjust men, how much more should we persist, despite the devil's delays, when dealing with our faithful Father. 

We can trust Him to always come through for us.