First of all, the genre of Genesis must be noted. It is critical to realize that historical narrative is the dominant genre of the Old Testament. This is no different in the book of Genesis, nor in the creation account in particular. While symbolic language is not outside of the realm of historical narrative, it must be recognized that the author of Genesis intends to communicate historically factual information. With that in mind, consider what a straightforward reading of Genesis 1 and 2 would produce. Would the idea that Adam and Eve are simply a select couple of hominids out of many even be among the options that one might consider reading Genesis 1-3 on its own merits? Unless one brought with them to the text the preconceived idea that Adam and Eve were not the sole human beings at the time, one would never conclude this. Now this isn't to say that it is therefore invalid and false, but rather to point out clearly where one's presuppositions lie. Someone who asserts that Adam and Eve are two among many hominids at the time have smuggled their personal bias into the Genesis account of creation. They are determined that the scientific consensus is correct and that therefore the Bible cannot mean what it seems to indicate. Their ultimate and controlling standard is scientific academia.
In addition, consider the vast theological implications of such a perspective. The biblical doctrines of original guilt and justification by faith hinge on the concept of imputation. Specifically, Adam is our covenant head and from him we inherit real guilt. Likewise, Christ is the new Adam and covenant head of all in him and therefore everyone in Christ inherits real righteousness. Our very salvation is tied to Adam as the forerunner and head of all of mankind. If he is simply one of many hominids at the time, how can he legitimately be the representative of all of mankind? In addition, are other hominids made in the image of God, or only Adam's descendents? These questions must be answered if one seeks to maintain a Christian theology while adhering to evolution.
Also a concern theologically is death before the fall. The biblical account insists that death is a result of the fall of Adam and Eve. The fall fundamentally changed the course of history and creation forever. However from the evolutionary worldview death is important mechanism of the machinery that propels life forms forward. If Adam and Eve were only two among many hominids on earth, then natural selection operated using death as an integral aspect long before Adam sinned. Is death an enemy that is finally to be defeated? Was this prehistoric world "very good" in the eyes of God? Again these issues must be dealt with if one decides to maintain both a Christian worldview and evolution simultaneously.
The fundamental issue is the melding of two contrary perspectives on all of life. Evolution and Christianity can no more co-mingle than can oil and water. Make sure that if you want to hold to the Christian perspective, that the authority of Scripture comes before the passing whims and theories of man.
I think that by and large Christians fear looking "unintelligent" to the scientific community. They want to be taken seriously and their ideas to be legitimately considered by all people, academics included. Denying something that is so fundamentally established by the "intellectuals" is perceived as a "Sunday-school" sort of faith where one has to check there brain at the door, OR be simply uneducated to begin with.
I think often times the intentions and motives are good. Christians don't want to place unnecessary stumbling blocks to people coming to Christ. They feel that this issue is outside the realm of "salvation" and therefore a side debate that should not be a hindrance. However, when one comes to Christ unbelievers must surrender all of who they are, intellect included. This not to say Christianity is unintelligent, rather the point is that submitting to Christ as Lord gives him authority in every realm, even my intellect. What you win them with is what you win them to (James White uses this quote a lot). I feel that giving people a pass at this point is a concession too far.