ReadyBoost: where is the catch?

The long established and rather bored USB flash drive market in recent months (notable for the release of Windows Vista, first for corporate clients, and then for the rest), has somewhat revived: More and more devices began to appear with support for Vista's proprietary technology - ReadyBoost.

About ReadyBoost on the official website write like this:

“Windows Vista can use USB flash memory as an additional memory resource for caching data, which speeds up read-write operations and improves system performance. [...] The use of USB drives common today as an additional RAM makes it possible to expand memory and increase computer performance. ”
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It's all good, but confusing some circumstances.
Firstly, the idea of ​​expanding RAM by using disk is not new - this is a familiar swap. The hard disks used for it, with all their speed, cannot be a full-fledged replacement of RAM: the data exchange rate is noticeably lower, the constant swap slows down the work. How can a flash drive speed up when there is a shortage of RAM, if flash drives are ten times slower than hard drives? For example, the recently announced Kingston flash drives have a write speed of just 3 MB per second.

Secondly, the question of reliability arises. Flash memory is designed for a limited number of rewrite cycles - something around ten thousand, if memory serves me. When using a flash drive under ReadyBoost, with constant data recording, this resource will come out very quickly.

In general, somewhere we are fooled, but I do not understand where ...