It occurs to me that back in the day you used to have to add all sorts of expansion cards to computers to get what you wanted, whereas now, most people, I think, run with very few cards at all. Chances are now that a desktop system has cards that override devices that are built into the motherboard: video, audio, network, etc.

In the days of yore, if you ordered a preconfigured system, you probably weren't going to get the expansion cards you wanted, which meant that you were going to have to discard a good portion of the system you got.

Now, though, even if you build it all yourself, your motherboard is going to have all those baseline devices built in anyway, which means that you're not overspending by buying the base preconfigured system. Plus you get the fact that it was tested as a unit and you get some support on top of that. Also, Dell, et al., are doing a really good job with cases these days.

</rationalization>