Last weekend my provider went down for several hours. For the moment my Frontpage site is running on an NT-server (without mirror) and, according to my provider NT means frequent crashes (suprise-suprise).
I was happy with the thought that at least the educated or experienced Google searchers could still find my information in the cache.
We have trainees in our company writing content for our website and doing research on internet. When they are new, I "teach" them how to go about using Google. I am amazed at how many time I have to use the cache because sites/pages are unavailable or extremely slow loading. (the highlight feature you mention is yet another reason for using the cache).
I am very pro caching, its a pity Google does not add the date stamp though and that you have to use side-tracks such as:
to find them.
I would even applaud Google, or for that matter any other search engine, caching every update if they had the storage capacity - and if they gave the option of deleting old caches at choice.
It would give fantastic copyright protection facilitation.
I would even be willing to pay for it and it could give other opportunities as mentioned in this thread: