從「You can now try merged /usr in Debian」這邊看到，早期因為硬碟空間比較小的關係，會切成 /bin、/usr、/lib 之類的目錄：
The original impetus for requiring these directories was due to space limitations in the first Unix implementations, developers favoring the change point out.
這個理由很明顯已經消失了，所以 Debian 就規劃要整併起來...