-1 ... because today's pcs are built out of fewer components (many things are on the cpu die for example), there is very little to customize in a standard setup that *really* makes a difference - which is why so many oem are going out of business, they've simply lost their selling points.
On the other hands buying a complete pc ensures you support (fewer points to argue over who has done what wrong) and people who do this for a living imho simply are better at building a clean machine with proper cabling than me doing it every so many years. Plus many oems get very competitive bulk prices for their components which I find very hard to match even when buying at the cheapest discount/online shops I know.
It might make a difference for hardcore gaming and ultra-high end setups, but for your general lightroom/ps editing personally I'm leaning towards buying a complete desktop once I feel the need for more speed than my current crappy laptop can deliver.
-1. Just because the chipset includes more features, it doesn't mean that they perform well. The cheap complete PCs have older/slower technologies, sometimes several generations older. RAM in groups and speeds that make them harder to upgrade to significantly larger values, power supplies that can barely support additional hardware (i.e. discrete video), etc. If you want a barebones computer (around 500), then a complete PC might be worth it, although you can still get chassis + MB + cpu as a package and add on for a competitive price).
The key to building your own is to get quality parts the first time. Reuse your monitor, keyboard, mouse, case, optical drive, HDDs and power supply. Upgrade the video card, motherboard, add a HDD or a SDD or whatever you want. The computer companies may be able to buy parts for less money but then you're paying for their labor, their overhead and their profit.