I'll answer anyway since it's an interesting question.
Space is still cheaper on hdd's, and data volatility concerns. Is the latter still an issue with SSDs, or have they reached the point now where you can store your music collection on them and not worry about losing it in two or three years?
With Time Machine, there's no reason for anyone on OSX not to have backups, it's so incredibly easy to do, and you set it up once and forget about it until you need it.
My original macbook has an SSD in the optical drive slot with all applications and the OS. In the HDD bay is a large hard drive containing all user data.
Fun oddity after switching to SSD: If the laptop gets REALLY hot from long term summer usage, in the past I would just slap a cooling pad under it to get the fans to stay quiet. Now, the cooling pad somehow causes the SSD to freeze, so I have the cooling pad sitting only under the left half of the machine, and all is well.
Don't answer, it was rhetorical.