To achieve equal performance on a write IOPS basis utilizing hard disk drives would require over 400 drives, 36U of rack space and consume more than 300 times the power.
If you'd rather not have some wet-behind-the-ears Victoria's Secret Webmaster pondering your camisole predilections, you can tell your browser to refuse cookies or to prompt you each time a Web site wants to write one to your hard disk.
Write caching could allow the hard disk drive to stay in a lower power non-spinning state for longer periods of time, extending battery life for the notebook computer.
Perhaps the biggest gain may be in dual storage computers where a flash memory device is used for storing operating systems, software and frequently accessed content (and maybe even for write caching) while a hard disk drive provides much less expensive mass storage of user data.