would Microsoft allow or document a feature that would cause harm to the Flash device?
No. Some people have the incorrect and mistaken idea that Readyboost works by storing the swap file on the flash drive. This is not so. Readyboost does not exercise a flash drive in the same way that using it as a swap partition would.
Flash memory can only take so many writes. Even with techniques such as wear levelling, swap file use will destroy a flash drive in a fairly short time.
The core idea of ReadyBoost is that a flash drive has a much faster seek time (less than 1 millisecond), allowing it to satisfy requests faster than a hard disk when booting or reading certain system files. (System files, right? That don't change - write once, read many times) By the way, unless you have USB 2.0 and
you are using a top end pen drive or memory card ReadyBoost won't make a lot of difference. And even then it won't be anything to write home about.
A swap file on the other hand is constantly changing as memory is paged in and out. Furthermore, Windows uses the page file in blocks of 4096 bytes but flash memory usually has logical blocks of 65536 Bytes and whenever a 4K block is written the flash device has to erase and rewrite a whole 64K block. So pretty soon all the memory cells on the flash drive will be worn out.