Casey,
I'm afraid that you're comparing apples and oranges here. You can't apply what Scripture teaches about one element of worship to another element of worship. If you want to argue that only the Lord's Prayer should be prayed, then present your argument and see it crumble when compared to the teaching of Scripture. You seem to be the one ignoring the arguments that have been presented to you.
Actually, I believe I'm simply consistently applying the EP argument to the worship element of prayer. I haven't quite seen my argument crumble yet, and there have thus far been a lot of posts on this thread.
Paul tells us in Romans 8:26 that the Holy Spirit aids us in our prayers because, "for we know not what we should pray for as we ought". If the Lord's Prayer is the only prayer to be prayed, then we would not need the Spirit's assistance in reciting it.
So, the Holy Spirit does not assist us in our prayers? That's an interesting argument. Anyway, if you read Rom. 8:26 in it's context, it is clear from v. 27 that "He [the Holy Spirit] makes intercession for the saints according to the will of God." Yes, the Spirit assists us in our prayers -- but that assistance is no different in our singing. The context is showing that since we do not know what we ought to pray for, the Spirit prays on our behalf! What you're arguing here really does nothing to the essence of my argument.
Paul does instruct us to, "Let the word of Christ dwell in you richly in all wisdom; teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord." in Col 3:16. Uninspired hymns are not the word of Christ, but words of man offered up as Cain's unacceptable sacrifice was or the strange fire of Nadab and Abihu.
Hmm. I could just as well argue that there isn't necessarily a parallel between "the word of Christ" and the "psalms and hymns and spiritual songs," because I can't quite remember Christ composing any "psalms and hymns and spiritual" songs for me. (Of course I'm not denying the role of Christ's Spirit in the inspiration of all of Scripture, but you see your argument isn't absolutely necessary in the context of this verse being written in the NT.)
And now you've just characterized my worship as pure idolatry. Thanks. Why don't you tell me what you
really believe!

This connection between "Cain's unacceptable sacrifice" and "the strange fire of Nadab and Abihu" with the uninspired songs that I sing is
your connection, not
Scripture's connection.
Another example I could give you in Scripture does address both prayer and singing psalms in the same verse. Jam 5:13 "Is any among you afflicted? let him pray. Is any merry? let him sing psalms." Now James does not instruct us to pray the Lord's Prayer, but he does instruct us to sing Psalms. He doesn't say, Is any merry? let him sing, he says sing psalms.
Please, do note: He does not say that we may sing only psalms -- in fact, Paul had written that we may sing psalms,
hymns, and spiritual songs! (Which, by my estimation, are not psalms! And you are free to disagree.)
Anyway, if your argument is that since James says we ought to "sing Psalms" and therefore we may only sing Psalms, why not consistantly apply that logic to prayer when Jesus commands us: "When you pray, say: Our Father, who..." You see, you can't have it both ways.
We have no example of Christ or the Apostles singing anything other than the Psalms of David in Scripture, but we do have examples of them praying prayers other than the Lord's Prayer.
Which is, after all, a pure argument from silence when it comes to singing. That they prayed other words is easily answered with this: they were inspired.