Rebooting is rare compared to regular use.

The box I'm writing this on has an uptime of 148 days, the last time it went down was to put a second graphic card in it. I really don't see how a 'faster boot' would be of any benefit, in fact any benefit in faster boot time would have to be balanced with how long it took you to install that drive, format it, install your OS and so on in the first time.

Use cases should center around that which happens frequently, optimizing boot time for anything other than a netbook or a laptop that you don't 'sleep' is a case of premature optimization.

For all the other use-cases you sketch main memory is much more effective than a cache in a device at the other end serial link.