The pro-instrumental folks generally point to what they feel is the ambiguity of Paul's charge to the Ephesians or the use of instruments in the Old Testament as their support. I recently had a conversation with someone who suggested that the use of instruments in the OT is not support for instrumental worship because we're no longer under the old law. That would certainly be a valid point if the use of instruments was commanded in the Torah, but it's not.
I'm neither pro nor anti-instrumental music. I don't believe there's enough scriptural basis to suggest that the use of instrumental music is sinful and/or forbidden in worship. Having been raised in a rather traditional CoC congregation, I prefer a capella worship, but I believe it's just that--my preference. Because I believe the issue to be a matter of preference, I feel that one's motivation ultimately determines whether instrumental music is right or wrong. If a congregation chooses to employ instrumental worship in order to allow members of the body to utilize their talents in praise to God, then play away. However, if the use of instrumental music would create a schism in a church or drive away droves of people, then I don't see the reason to use instruments in the name of progress. At this point, a pro-instrumentalist may be shouting, "But if it's a matter of preference, then I should be able to do what I want to and other people just need to deal with it or leave!" This is where I believe our American sense of independence taints our faith and conflicts with our call to humility and unity.
Whether or not the use of instruments would be to the detriment of the unity of a church isn't the only matter to consider. Too many churches have adopted instrumental music primarily to appear hip to the culture. There's a distinction between being culturally relevant and culturally driven. One of the largest churches in Austin (not a CoC) airs radio spots promoting their Sunday morning worship as basically a God-friendly rock concert. To reduce our worship to that level not only woefully reduces the significance of the act, it also conveys to non-believers that being a Christian is basically just praying or singing to God occasionally while happily pursuing coolness and our own ambitions. That sort of self-seeking mindset, which pervades Christianity all too much in America, is the antithesis of Christ's call to His people. A church that is seeking to be more culturally relevant will seek ways to minister to their community, to meet the needs of struggling people, not to entertain them.
I suppose my hope is that at some point, CoC's will agree to disagree regarding instrumental worship, for the a capella folks to offer praise but not condemnation, and for the instrumentalists to glorify God but not our culture.