Turning a computer PSU into a scientific power supply gives you a cheap, well regulated supply with the disadvantage that voltages aren't adjustable. They're perfect for working with microcontrollers and such as you get nice, stable 3.3V, 5V, and 12V rails with usually a +/- 10% variance at most.
A real scientific bench power supply is much more expensive, has the advantage of adjustable voltage, but generally are rated for relatively low amperage. The voltages are spot on
, with +/- 1% variance on a good supply. That sensitivity is attained by basically ignoring efficiency - they do whatever it takes to get you a stable current.
So, unless you have a bunch sitting around like you do, and don't need to care about efficiency, it's generally not worthwhile to go in the other direction