I hold to the Regulative Principle of Worship, but do not hold to exclusive psalmody (since the Psalms themselves are not exclusive "sing to the Lord a new song...") and find the rationale towards it a hyper-scrupulous interpretation of the Regulative Principle - that is "what is not commanded is forbidden" - the problem is that there is a corollary that is often ignored - "what is commanded is required" - the hyper-RPW'er spends so much energy following the first rule that they ignore the second, to the exclusion of the liberty the New Covenant believer is given in this area.
A brief apology for Inclusive Psalmody:
Colossians 3:16 (New American Standard Bible)
16Let the word of Christ richly dwell within you, with all wisdom teaching and admonishing one another with psalms and hymns and spiritual songs, singing with thankfulness in your hearts to God
This Scripture is a cornerstone to defining the RPW in terms of music to the Lord's worship.
The reasoning goes something like this:
"Let the Word of Christ dwell richly within you, with all wisdom teaching and admonishing one another.."
This obviously directs us to fullness of God's word as our ultimate authority and source of all wisdom - including the source of our worship in song - sola Scriptura. In this the IP-EP debate is in full alignment.
"with psalms and hymns and spiritual songs..."
This is the crux of the IP-EP debate - we are perfectly able to understand how we may worship with psalms; there are 150 of them right in the middle of the Bible (coincidence?).
This, to both sides, leads to the next question - "How then, holding scrupulously to the RPW, are we to worship with hymns and spiritual songs?"
Many EP'ers look at this and concludes, in summary, "Since there is seemingly no clear exposition in Scripture of these 2 additional types, following the analogy of faith, one must conclude that this is a reinforcement of the first object - that is - the rendering could easily be "with psalms, psalms, and psalms" - exclusively.
The IP'er (at least this one!) - rejects the EP interpretation and instead looks at the same passage and concludes, "since we are commanded to sing the 3 types of songs, how best may one fulfill the commandment, align to Sola Scriptura and not dishonor God by worshiping with "strange fire" (Leviticus 10:1-3)?
The rationale is thus: Since the NT does not go into detail concerning specific templates for hymns or spiritual songs, then a) there is some liberty in the composition of these songs (much as there is liberty in the composition of prayers and sermons) and b) we should search the Scriptures to see if there is clearer guidance for our praxis in this area.
And, oddly enough, the Lord has been gracious enough to deliver to His Church a source for these templates - the 150 Psalms. That is certainly not to exclude the fullness of Scripture as source material, since the Psalms themselves utilized the then available Scriptures as content sources, but certainly to look to these inspired songs, or psalms, to guide us into God honoring and newly composed hymns and spiritual songs.
Thus may we align to the fullness of the RPW and, in our practice and understanding, have confident assurance that we are singing with thankfulness in our hearts to God.