Considering that the chips used for the root partition are only now crossing 1GB, it will likely still be done if you want a large number of applications on your device. No other phone does what the N900 does, and Android has to be rooted before you can run apps from the SD card (which is essentially optification.)

It lets you one-source all applications and lets you maintain a cleaner, easier to update system. Dependencies and conflicts become easier to manage and avoid, allowing Nokia to do the SSU push of PR1.1 without forcing everyone to do a full reflash.

Package management is a great thing, it's only unusual or of questionable value to those whom have never used a system based around it.