Not such a great idea. SSD R/RW speeds are in the mid 500MB/s, while even the fastest USB 3 flash drives are between 1 and 200 MB/s . Not to mention speed lost due to using the USB bus vs the SATA bus.
Edit: The USB 3 drive I used for comparison was over $200, which is the same price as a 256GB 840Pro SSD. What USB drives were you looking at?