It's not a salvation issue. Some say it is, but even those people usually say that it is only a salvation issue due to where it leads (see below), and agree that it isn't a salvation issue in and of itself.
There are two questions there. For a debate to happen, the issue has to be cared about by both sides of the discussion (otherwise, one side says "meh.", and the discussion is over.). So you are actually asking two questions: 1. Why do evolution supporters care? and 2. Why do creationists care?
After being in these discussion for over 20 years, and seeing all the arguments (from both sides) many times over from many different people, posting literally thousands of messages about this topic (and reading dozens of thousands of posts on it), the answers to both questions above seem pretty clear to me. So here are my answers.
First, the easier one - question #1. Why do evolution supporters (like myself) care to discuss this?
Because evolution is not only supported beyond a reasonable doubt by evidence, but is critical to many of the technologies that run our modern world. By denying evolution, one prevents oneself from being able to contribute in many areas, and hence to have a good job. Thus teaching evolution denial to kids often robs them of a secure future, and thus hurts our society and everyone in it. Evolution denial also hurts Christianity, making the Gospel look like willfully ignorant mumblings, as St. Augustine pointed out centuries ago. I'll leave it at that, because I suspect you are more interested in question #2.
Question #2. Why do creationists care to discuss evolution?
Because they don't see Genesis as "looking like" or "intended as" a metaphor. Thus, as the word of God intended literally, accepting evolution would mean intentionally and knowingly rejecting God's word. If that's done, that's blasphemous - regardless of evidence. Or, even if it's unclear whether or not it is intended literally, rejecting a literal reading of any scripture that is not clearly non-literal call into question all other scripture, allowing it to be read non-literally. So what's next? Christ's miracles were non-literal, so they didn't actually happen (though in John's Gospel they are routinely used as proof of Christ's divinity - so then Christ is no longer divine?)? Then we stop seeing the resurrection as literally true? Is there anything left of the Christian faith if one can take any and everything as non-literal?
Creationists, if that's not your main concern, or if I misstated it, please correct me.
So there's your answer.
Regarding question #2 (you main question), let me add my own thoughts.
I think the concerns raised above can be (indeed, must be) ignored by any Christian today. Why? Because that train has left the station long ago. Christians today already take large portions of scripture non-literally - portions that have a stronger claim to a literal reading than Genesis. Christians today can already read larger portions non-literally without any problem with calling fundamental doctrines into question.
Which sections? Many. Some examples:
Literally dozens of sections of scripture make it undeniably clear that a literal reading of the Bibles gives a flat earth under a hard sky dome. There is a much stronger case for biblical globe-denial than there is for evolution denial. You can see some of them in post #32 here:
http://www.christianforums.com/thre...ustify-not-being-flat-earthers.7926035/page-2
Jesus' return by the 3rd century. There is a much stronger case for Jesus' return before 200 AD than there is for evolution denial. Large sections of scripture, and many of Jesus' own words, are taken non-literally by nearly all Christians today on this topic.
Acceptance of slavery. There is a much stronger case for biblical support of slavery than there is for evolution denial. Large sections of scripture - including the 10 commandments, are taken non-literally (or just outright ignored) by nearly all Christians today on this topic.
A lower status (as property) of women. There is a much stronger case for biblical sexism than there is for evolution denial. Large sections of scripture - including the 10 commandments, are taken non-literally (or just outright ignored) by nearly all Christians today on this topic.
and so on. In the near future, as we move past many other outdated beliefs, new sections will join this growing list of topics where we Christians move past a literal reading of our scripture. Some of these are already pretty clear. Within a few decades the condemnation of homosexuality will likely be seen in the same way as slavery and sexism. The same goes for spanking children. Adding evolution to the list above will be easy and inevitable - especially since one can make a good case for a non-literal reading (much more so than any of these other topics).
In Christ-
Papias