If you read through the link I posted earlier, you'll see that the hard drive is still used even if the data is also on the flash cache:

Q: Aren't Hard Disks faster than flash? My HDD has 80MB/sec throughput.
A: Hard drives are great for large sequential I/O. For those situations, ReadyBoost gets out of the way. We concentrate on improving the performance of small, random I/Os, like paging to and from disk.

And everything is written to disk anyway:

Q: Isn't this just putting the paging file onto a flash disk?
A: Not really - the file is still backed on disk. This is a cache - if the data is not found in the ReadyBoost cache, we fall back to the HDD.

As per my earlier memory pressure comment:

Q: How much of a speed increase are we talking about?
A: Well, that depends. On average, a RANDOM 4K read from flash is about 10x faster than from HDD. Now, how does that translate to end-user perf? Under memory pressure and heavy disk activity, the system is much more responsive; on a 4GB machine with few applications running, the ReadyBoost effect is much less noticable.