The main utility of CLUSTER is to mark a table as sorted - that is, few disc seeks will be required on an index-scan of the table. This is important when doing (for example) a merge join with another table, or just when you are requesting a large proportion of the table in sorted order. Postgres knows enough statistics about the order of entries in the table to know that it can read the table faster in order using the index with the occasional seek for recently added elements than if it was to do a sequential scan and re-sort the table in memory (spilling to disc if necessary).

A B-tree can in no terms be described as being laid out on disc in primary key order. The individual pages of the tree are placed on disc randomly, as they are allocated. Therefore an index scan won't return the rows in index order as quickly as the current scheme of having the rows separate from the index and sorting them every now and again.

Ultimately, for the goal of fast in-order scan of a table while adding/removing rows, you need the rows to be laid out on disc in that order, so that a sequential scan of the disc can be performed with few seeks. This requires that inserted rows are actually inserted in the space they should be, which is not always possible - often there isn't space in the page, and you don't want to spend lots of time shifting the rest of the rows rightwards a little bit to make space. To a certain extent Postgres already does insert in the right place if there is space in the right disc page (from deleted rows), but because this is not always possible, the solution is to re-CLUSTER the table every now and again.

I think the Postgres way is actually very well thought out.