SSDs are still relatively expensive, while SD cards are coming way down in price. Two 32 GB SD cards with a USB 3.0 interface can be had for about $60, while a 64 GB SSD is about double that. Would a 64 GB SSD with SATA 3 perform enough better to justify the price difference? Any good reason not to install my OS on a spanned 2-SD-card dynamic volume?
Most SD cards are very slow compared to an SSD. You can buy fast SD cards if you pay enough money, but by then you're into the range of SSD costs anyway.