I thought about doing that too, when I bought my B2. So I read up a little on it. The primary advantage of an SSD is faster access time. That's really nice when you're waiting for your computer to boot up. The B2 only takes 10 seconds to boot, and the OS is on the SD card, not on the HDD. So an SDD would provide no advantage in that respect. When it comes to reading the music data, faster access would be imperceptible - an HDD can read the music plenty fast to keep up with the music.
There are drawbacks to an SDD, as well. First of all, they are quite a bit more expensive. That may change in the future, but for now, that extra cost doesn't get you much bang for your buck on a B2. Also, as I understand it, the life expectancy of an SSD is generally shorter than a (non-defective) HDD. Albeit, an HDD can fail at any time. I suppose an SDD could, too.