if Google update PR every day then we can see new PR every day.
Isn’t that good!
wisely to understand how PR relate to your site maybe very helpful for SEO
if your site always keep PageRank PR = 0 , it may tell your site has poor traffic most of time.
what if your site have very high PR but no traffic at all, it may tell you that your site may be poor of contents and not attracting people to come back but may only have a lots of backlinks.
PR changing may tell you that some of your site backlinks are moving
example: some dp threads have PR3 this time but next time it may become PR0, it could because these threads have been moved off from the first page of forum.
So PR might be still providing some information. It is not total useless.