Cheap poor quality power supplies are really for people who do not really understand about computers, and the quality level of power supply that is needed, for long term, reliable operation.
It's like buying new dishes at home, made out of paper, because they are cheaper.
You need proper china or whatever dishes, otherwise they will break/wear-out, very quickly.
The poor output voltage accuracy/stability/ripple may even contribute to breaking stuff earlier than necessary in your computer, such as HDDs, DVD-RW etc.
If you are buying VERY cheap stuff, it is best to make sure that you do not cheap out, in ways which will make your computer potentially unstable/unreliable/bad.
Anyway it's your money, and your choice.
EDIT: Some people have to make do with the cheap PSUs for various reasons. By making sure they use massively less than the rated load, not using the computer that much, not using high current graphics cards, and changing the capacitors in them with a soldering iron (if the capacitors break). They have coped.
But I would still say, get a proper power supply.