Let me try to draw an analogy between scriptural interpretation and mathematics (since that is where my training lies).
The mathematics that we are most familiar with (that of the Real Numbers) relies on 13 postulates of the Complete Ordered Field — one might call them a set of presuppositions. From these initial assumptions, all the rest of the system is built.
Mathematicians do not KNOW that the postulates of the Complete Ordered Field are consistent. They cannot prove that there are no contradictions in these founding principles. They are assumed to be consistent (inerrant) and rather taken on faith (and the fact that mathematics has generally proved to work well over the years).
Given the assumption of consistency, mathematicians are free to do mathematics, and prove theorems using the axioms and postulates with logic. A mathematician, in producing a proof, never justifies his result by trying to show that the result is consistent with *all* the postulates and axioms in the system. It doesn’t matter WHICH postulate or axioms are used in the proof. They all guarantee the same reliability of result.
If scripture is presumed to be inerrant, and in particular internally consistent, then one is justified in “doing theology” in much the same way that mathematicians do mathematics. One may take any part of scripture and apply rigorous analysis to it (including cultural and linguistic analysis), apply logic, and reach a guaranteed valid result and may do so without having to justify the conclusion by comparing it to every other part of scripture.
I don’t mean to suggest that every inerrantist puts blinders on and doesn’t look at scripture more widely, but the inerrantist assumption certainly gives one a license to do so.
I recall an e-mail debate I had with a fellow earlier in the year, who argued that homosexuality was a punishment from God (based on Romans 1:28). He wasn’t interested in other parts of scripture, or modern psychological theory. He had a text; it was inerrant; and that was sufficient for him.
Dropping the consistency assumption forces one to work much harder to make a point based on scripture. One could point to other parts of Scripture about how God is merciful. One could point to psychological studies that show that sexual identity is determined in childhood.
I am reminded of the argument of one of the Church Fathers that infants should be baptized because they are helpless, just as helpless as adults are without God’s grace.
If we make the inherent assumption, then we are saying that we are capable in and of ourselves (using a text and logic) to know God and that we can do so without any teaching from the Holy Spirit.
This is the first reason why I do not think that the doctrine of inerrancy is good for the Church.