If I dispose of an old PC it normally means that it will never work as a whole and anything that still does work gets kept as "spares".
If it ain't broke, don't fix it. Why buy a brand new system if all you need is more Ram or a new graphics card? Need more space? Buy an extra drive. No spare IDE/SATA connections? Buy a controller board. No space for that? Buy an external drive.
If you HAVE to have a new PC but the old one is still fine, then you have a second PC to use for backing things up or, if you are in a house like mine with two teens at college, another PC you can use for browsing/documents until it explodes. Makes sense to me.