ReadyBoost was a bandaid for the early Vista systems that came from XP with only 512MB ot 1GB of RAM. Thoses systems were suffering from a lot of page thrashing (high paging activity). In those cases a USB stick that had a very good access time (less than 1ms) could help speed up the paging. Problem is that most USB sticks are too slow to make any difference anyhow.
Compare the access time of the 2 sticks below - both Kingston Traveler. You can probably imagine that the first one would be completely counterproductive. A HDD is 4 times faster.