Oh ok... didn't think of that because it was discussed in this thread
Advantages:

- Backup system on flash. If you brick your OS you can use your backup system.
- Faster booting. Jffs2 uses compression, so there is a big overhead which results in slower transfer rates and cpu usage. The filesystem on on your SD card can be ext2/ext3 etc.
- Extends your rootfs and you can install more than 256mb of applications.
- Testing system. Install an application on sd and test it. If everything works you ca finally install it on flash. So you can have a cleaner OS.
- ...