that's a double edged sword though. if you don't have enough spare cpu, you won't be able to handle any sort of spike in usage. say from an advertising campaign that works a little too well or if a server goes down.

and once you start maxing out boxes, the load comes even harder since the site loads slower. you very quickly run out of resources and suddenly you're SOL and everybody's mad that the site won't load.

honestly, it's hard to point at any one source of problems when scaling a site. cpu, ram, and i/o are all equally problematic. and if you under plan any one of those it can take your site down into a spiral of doom.

you really must allocate your resources for peak load, not for average or nominal load. and then allow for some failures.