What if google Pagerank update every year not every 3 months? What do you think will happened to the Link Sale forum here..? For me its ok as long as SERP updating often.
Alexa has a limited user base, technical and or web business people. I use it to find sites that I am competing with. PR created a nice cottage industry and gives Google the mindshare in that industry. It's a measure that people use for selling links and sites. I think Google probably likes the fact that people focus on PR even if it causes them some work to keep it from being gamed too much.
Maybe that forum will be full of expired domains offers !!! IMO the best PR update period would be monthly or every 2 months.