I was working on a bunch of PC PSU organ donors and noticed that on lots of them through all the wires that lead the power to the outside there were wires soldered, and lots of solder added. This is probably to increase current capability.
I only today thought about it, so did not take any photos, but it looks similar to this: 
Now an ATX PSU looks quite crowded and there is probably not enough place for wider traces, so I was wondering if this is acceptable/good design or if I should avoid it and go for proper traces where I can; and why should I do what.
Best Answer
Soldering wires onto a PCB like that definitely increases current capability. Whether that's good design we can't of course answer without knowing the design criteria.
Downsides of this method are that it takes significant manual labor in production, therefore will reduce yield because people make mistakes, cause more unit to unit variation than othewise, and won't be as corrosion resistant because there won't be a layer of solder mask over the conductor.
Whether these tradeoffs are "good" or not depends on factors we can't know. For example, if this board will be manufactured in a part of the world where you can pay someone a bowl of rice and $.50 a day, then it makes a lot more sense than where you have to pay US minimum wage.