Well, there are other compelling reasons to build your own too, e.g. specific configs. Dell *definitely* doesn't offer every part I'd want - not by a long shot - and the places that do are very high priced by comparison. You can also save money if you have parts lying around, like I do, and have certain kinds of systems to build. For example were I in Zaine's position I would buy a new CPU, motherboard, memory, case, and power supply and either a motheboard with graphics onboard, or a cheap graphics card. Then I'd supply hard drive, CD/DVD-ROM, keyboard, monitor, etc. from spares I have around. I've got about 15 spare hard drives >120GB around, and that should be more than enough for your average mom cmputer. But most people don't have that kind of hardware sitting in their closet.
So generally I agree, for the majority of people the custom-build can do more harm than good.