These two major parts of your computer are generally the hardest hit in the war against dust. Needless to say that extreme air cooling where hundreds of cubic feet of air per minute are forced in and out of your pc increase the intake of dust ten fold. There are several reasons. From a vain standpoint, If your PC is decked out with windows and lighting then you want the inside to be nice and clean for showing off. Granted this is pure vanity but seeing a nice clean PC inside shows that the dust is kept under control or at least is regularly maintained.
More serious reasons stem from the component cooling.
Along with the all important CPU, Video cards and even mobo chipsets are using active cooling to keep temperatures down and increase performance. Left unchecked these areas are going to continuously build up with dust to a point where the cooling is no longer efficient. Dust actually acts as a semiconductor and excessive amounts eventually clog up fans which will eventually stop working altogether causing hardware failure. Cleaning our your case on a regular basis very much helps keep dust levels to a minimum. I don't really worry about it.