Proper Use of Proof Texts
Biblical proof texts derive a great deal of their meaning from the larger context (or "metanarrative"...5 more Emergent points for me!) of the entire Bible. The larger narrative (the story of God's redemption of human kind, for example) provides the context in which to understand a single verse. However, because this larger narrative is composed of individual verses, each verse (potentially) contains a point of truth for the whole narrative, and thus (potentially) a single verse can be cited as stating one particular truth. So the value of proof texting is that it can concisely point out one particular detail of the overall narrative. The danger with proof texting is that it is not terribly valuable in apologetics or polemics, where someone is likely to reject your understanding of the narrative, since often the over-arching narrative is important for interpreting the proof text in question. Unless both parties are operating within the same narrative, it seems likely that they can completely disagree as to the point of a proof text.
To use an example, when the Westminster Confession says "Not only those that do actually profess faith in and obedience unto Christ, but also the infants of one, or both, believing parents, are to be baptized," and cites Genesis 17:8 in support ("And I will establish my covenant between me and thee, and thy seed after thee in their generations, for an everlasting covenant, to be a God unto thee, and to thy seed after thee"), this verse is only going to provide support for infant baptism to someone who accepts the "narrative" of covenant theology. A dispensationalist can affirm the truth of this verse, but disagree about its impact on baptism within the church. If someone's over-arching narrative differs from yours, and they can twist any verse to fit within their own understanding of the narrative, the proof text will be useless in proving your point.
All this to say that I think proof texting should be used when working with someone with whom you share the same understanding or "metanarrative", and polemical debates are better resolved by presenting the big-picture, metanarrative to the opposing party rather than proof texts.