Well, if you valued cake you'd want a way to talk about cake and efficiently distinguish cakes from non-cakes—-and especially with regards to planning, to distinguish plans that lead to cake from plans that do not. When you talk about cake there isn't really any reification of "the platonic form of cake" going on; "cake" is just a convenient word for a certain kind of confection.
The motivation for humans having a word for goodness is the same.
I don't necessarily have a problem with using the word "good" so long as everyone understands it isn't something out there in the world that we've discovered-- that it's a creation of our minds, words and behavior-- like cake. This is a problem because most of the world doesn't think that. A lot of times it doesn't seem like Less Wrong thinks that (but I'm beginning to think that is just non-standard terminology).
There seems to be a widespread impression that the metaethics sequence was not very successful as an explanation of Eliezer Yudkowsky's views. It even says so on the wiki. And frankly, I'm puzzled by this... hence the "apparently" in this post's title. When I read the metaethics sequence, it seemed to make perfect sense to me. I can think of a couple things that may have made me different from the average OB/LW reader in this regard: