I think the OP misunderstands readyboost a little bit.

Readyboost lpre-loads commonly accessed files on to reserved space on USB flash drives. Only USB 2.0 drives are supported, and some flash drives that are 2.0 may still fail due to having inherently low speeds.

The reason that these small files are loaded into the flash drive is because the access times are much faster than that of a hard drive. In a hard drive, the read/write head has to physically move around the platters inside the drive to access information stored in different locations. On a flash drive, it's all solid-state, so access times are considerably faster - no moving parts to have to wait for!

Our phones don't have hard drives, their internal storage is solid state. The internal memory and RAM both have fairly high data transfer rates... especially when compared to the transfer speed of the SD card slot. So because there is little-to-no latency for small, random read/writes, something like readyboost wouldn't help at all.