Tomzo, I know i am risking receiving you fury but frankly it's a risk i am prepared to take. Page rank gives an idea of how important/valuable google thinks a page is. If the page rank did not change more frequently than it does on the toolbar then the search results would stagnate.
Lets take a look at the page rank equation (google actually uses an unknown derivation of this)
PR(A) = (1-d) + d(
PR(t1)/C(t1) + ... +
PR(tn)/C(tn))
As tomzo is so clever, i wont need to define the different terms, but hopefully it is clear, that to run the results of the equation for every site just once every 3 months would be far less efficient than to run the results frequently, update them internally, but to provide stability only update them externally every 3 months or so.
I think i have made this overly complicated, something i dont like to do, so here's what you need to know.
Google updates
PR internally.