Many OEM computers (Dell, Gateway, etc.) use the fans in the powersupply as the major case ventalation. The reason the powersupply is running cooler is because it is cooling off with fresh air, rather than the warmer air of the computer. Many PSUs now have fans that adjust depending on the internal temperature of the PSU. So running your PSU outside your case allows it to run cooler and therefore quiter. If you do decide to use your computer this way, be sure to verify that there is enough ventalation fans to keep the warm air from building up inside the computer. Another thing you may want to be sure of is that the PSUs are attached in some way to the case, this way they cannot get bumped off and pull on the components they are attached to. One last thing to be sure of is that the case is still grounded. The case of the PSU is grounded, and when it is screwed into the CPU case the case becomes grounded as well.
Those are just a few of the important points to consider, but otherwise there should be no problem with running your PSUs outside of the case.