For most YECs it seems to be something fundamental. According to them, if you don't interpret Genesis 1-11 literally, then the rest of the Bible loses authority. (It's a strange corollary, isn't it?)
I guess different TEs have their different reasons to get worked up about it. For some of them creationism represents an embarrassment to the faith, and they want to obviate that embarrassment. I'm not much motivated by that. For me, creationism is a really interesting phenomenon that stems out of people's (incorrect, to me) understanding of how science interfaces with theism.
Take a normal complaint against evolution: "Evolution takes God out of the picture of how life originated, and therefore it asserts that man can have control over his destiny instead of being answerable to God." Now, you would rightly conclude that you shouldn't care about this. Whether we were formed from dust or evolved from slime, both are really quite undignified origins to me, so I don't see much spiritual difference between being specially created or being evolved. But let's say I changed the terms a bit: "Condoms take God out of the picture of whether sex results in a baby; it asserts that man can have control over his reproduction, instead of God." Now it hits home, doesn't it?
Science interfaces with theism increasingly in our world, where more and more is being explained on a physical level by science. Just because science is explaining more, does that mean science means more? Some of the atheists are convinced that this is true, and oddly some creationists can be interpreted to be agreeing with them in this respect. They discuss what that might mean in the context of old musty ape fossils, and weird rocks that a Flood can't explain, and about the very shape and filling of the universe that simply doesn't look 6,000 years old. That might not seem relevant. But if you watch carefully, the very same attitudes and arguments are being applied to stem cell research, and questions about the biology of sexuality and psychology, and about what it means to even have science in a theistic world. The arguments and techniques we develop here in OT have great ramifications for those.
So no, YECism itself simply isn't important or relevant to my spirituality. However, the questions that it does raise are very relevant.