I think the logical basis comes from saying we are in the 'church age' so only the writings of Paul about the rules for this age are 'directly relevant'. Other parts of the bible obviously help us to read the writings of Paul correctly.
See the table in this link for a summary of the different ages: http://en.wikipedia.org/wiki/Dispensationalism
The logic is:
- 'the fall'= we thought we could do it all ourselves
- 'the law' = a way to live that would bring blessings upon God's people
(but with only a few like Moses, Joshua and David being filled with
- 'pentecost' = through the actions of Jesus we are
all able to be filled with Gods spirit and walk humbly in the spirit
to do God's work.
As we are in the 'Church Age' and able to be filled with the God's spirit some people say Paul's writings argue there are no rules except that we walk humbly in the spirit seeking to do God's will. However, Paul's writings would also suggest we need to work at this and be subject to church discipline when we start failing to show the fruits of the spirit (depending on the church this might be through greed, sexual immorality, envy, gossip etc as shown in Paul's writings to various churches he had helped establish).
I think I would be with most Christians in thinking that all the books of the Bible have been recognised as being written by people with -at the very least- some great wisdom and understanding of the nature of God. However, logically speaking, some books are more directly relevant to our present lives than others.