Well, a business model that pays 50 cent a page view sounds nice for sure - I fear most of us don't share that luxury.
I figured that most sites who would be interested in such a database either fall into the retail (recommendation) or the community (social graph) category. Both operate mostly on volume and the last thing you want is a hard bottleneck just when you're at the verge of becoming successful.
But well, if your business model doesn't require scalability on the web-tier then yes, these concerns ofcourse don't apply.
I don't believe you'd put all of your information in this kind of custom db. The author remarks that they are running with memory-mapped disk back-end, which means a single machine should be able to take you pretty darn far.