Much like the philosophical and moral chaos that results from naturalism, all sorts of theological mischief ensues when we reject or compromise the literal truth of the biblical account of creation and the fall of Adam.
I realize, of course, that some old-earth creationists do hold to the literal creation of Adam and affirm that Adam was a historical figure. But their decision to accept the creation of Adam as literal involves an arbitrary hermeneutical shift at Genesis 1:26-27 and then again at Genesis 2:7. If everything around these verses is handled allegorically or symbolically, it is unjustifiable to take those verses in a literal and historical sense. Therefore, the old-earth creationists' method of interpreting the Genesis text actually undermines the historicity of Adam. Having already decided to treat the creation account itself as myth or allegory, they have no grounds to insist (suddenly and arbitrarily, it seems) that the creation of Adam is literal history. Their belief in a historical Adam is simply inconsistent with their own exegesis of the rest of the text.
[That hermeneutical shift is clearly illustrated in this post. Bruce Waltke is willing to dialogue about an evolutionary view of Genesis 1-2; Tremper Longman III is comfortable questioning the historicity of Adam and Eve.)
But it is a necessary inconsistency if one is to affirm an old earth and remain evangelical. Because if Adam was not the literal ancestor of the entire human race, then the Bible's explanation of how sin entered the world is impossible to make sense of. Moreover, if we didn't fall in Adam, we cannot be redeemed in Christ, because Christ's position as the Head of the redeemed race exactly parallels Adam's position as the head of the fallen race: "For as in Adam all die, even so in Christ all shall be made alive" (1 Corinthians 15:22). "Therefore, as through one man's offense judgment came to all men, resulting in condemnation, even so through one Man's righteous act the free gift came to all men, resulting in justification of life. For as by one man's disobedience many were made sinners, so also by one Man's obedience many will be made righteous" (Romans 5:18-19). "And so it is written, 'The first man Adam became a living being.' The last Adam became a life-giving spirit" (1 Corinthians 15:45; cf. 1 Timothy 2:13-14; Jude 14).
So in an important sense, everything Scripture says about our salvation through Jesus Christ hinges on the literal truth of what Genesis 1-3 teaches about Adam's creation and fall. There is no more pivotal passage of Scripture.
What "old-earth creationists" (including, to a large degree, even the evangelical ones) are doing with Genesis 1-3 is precisely what religious liberals have always done with all of Scripture—spiritualizing and reinterpreting the text allegorically to make it mean what they want it to mean. It is a dangerous way to handle Scripture. And it involves a perilous and unnecessary capitulation to the religious presuppositions of naturalism—not to mention a serious dishonor to God.
Evangelicals who accept an old-earth interpretation of Genesis have embraced a hermeneutic that is hostile to a high view of Scripture. They are bringing to the opening chapters of Scripture a method of biblical interpretation that has built-in anti-evangelical presuppositions. Those who adopt this approach have already embarked on a process that invariably overthrows faith. Churches and colleges that embrace this view will not remain evangelical long.