The reason I consider it an advanced feature is because if you want some function of a set of non-contiguous columns you can always, if necessary, build that function up in stages by using intermediate cells. (Which in this case leaves you with something very similar to sea_of_flame's suggested workaround.)

That said, it does seem like the kind of thing it would make sense for Docs to include on the basis that it's a relatively simple feature to implement which will be invisible to anyone not trying to use it, but very welcome for users accustomed to Excel.