I don't believe that any saw will chop a "perfect" miter every time. There are too many variables with the blades and the wood grain oddities to allow that to happen. I get some pretty good chops on my saws but I wouldn't say that all of them are perfect by a long shot.
The miter sander has some good points going for it though. Once it is calibrated to sand a "perfect" 45º angle, it has nothing to "run out", it doesn't move fast, and it is consistant in it's job. You can therefore true up a cut with an accuracy that isn't always there with a saw blade running at a couple of thousand revs. per minute.
If you are buying chops on a regular basis, I would recoommend buying a miter sander because, as someone said, the employees don't always share the feeling of quality that we share and sometimes they simply are having a "bad hair" day!
Framerguy