It would be one thing for the proponents of evolution to say, "Well, there are a lot of gaps in the evidence, and we're really not certain that the variety of species rose according to the classic theory of macro evolution, but it's the best thing we have going," but they don't say that. By and large, most evolutionists are strident in their views, and combative in their articulation of those views. It has nothing to do with "science." I think it has to do with the same reason behind the "DiVinci Code"'s popularity -- evolution gives comfort to people in their unbelief and rebellion against God. Now, having said that, I know of sincere Christians who believe in evolution. But most of them struggle to reconcile what they believe about evolution, and what they believe about God.
I know of what I speak, because at one point in my life (in the midst of my college education), I was an ardent believer in evolution. But I've come to see it as an empty husk of a theory, based not on evidence, but on wishful thinking.
It just doesn't make sense if it wasn't evolution from common descent.
And, no, "common designer" doesn't make "just as much sense" as common descent. It's nowhere near as tight a hypothesis. ("Common designer" where the designer is presumed to be omnipotent excludes nothing which can be imagined.) Furthermore, no sensible motive for "the designer" to do what he would seem to have done is yet apparent.