This is exactly like when you are analysing a living organism. If its evolution would be fast you could immediately see the differences between 2 different points and so create a logical scheme of how it works. Slow turn-arrounds guarantee that the Google algo is well protected against behaviour analysis.
"Fluctuating data will always be less credible because it doesn't give proper information all the time. One day your site can perform as a PR7 and the next day it can perform as a PR1, that would not give a good balance of how your site performs on average." Tremendous point.
Exactly its so people dont exploit the system as easily. Plus Google has to perform a deep crawl to extract all the data to evaluate page rank properly. This would be a drain on not only their resources but the webmasters servers as well. Although it would be handy to have Pagerank display a more accurate value such as PR5.9 instead of rounding it up or down to the nearest whole number.
I think I read somewhere that it is a very resource intensive process and thus only performed intermittently. The reason is that the scale is relative. A PR 4 only has meaning in terms of it's relationship to a PR3 or PR8, etc. So it doesn't seem to me anyway to be something that they could do on a partial or rolling basis as the entire PR system gets updated and then mathematically adjusted to fit on their 10 point scale.
As far as my experience goes (I'm a newbie with only 64 sites online since 1993), Google updates in two ways, mass updates and selective updates. I see pages of mine updating all the time in PR and backlinks but honestly, I'm only concerned with my SERPS. my three biggest tips/tricks I could tell any new blogger is #1 - build good content #2 - Use social networking sites like Digg, MyBlogLog, BlogCatalog and ALWAYS add a Feedburner link to your site. #3 - Use TAGS - especially Technorati tags and use UltimateTagWarrior. Oh no, I think I just gave away the best tips from all those crappy E-Books you might buy on Clickbank....
also..it could be that since there are so so many sites..it takes time..not that they cant do it.. i think its more of letting us wait for it...live PR updates would make people go in sorta frenzy ;-)