No motherboards are not designed to run on dirty power, they are designed to run on ATX Spec Power supplies, and ripple is indeed part of the ATX spec.
Its no coincidence that people with apparently "decent" PSU's find PC's with high end graphics cards are often unstable. Levels of ripple outside the ATX spec can and do cause instability, and reduce the lifespan of electrolytic caps. (Although many high end motherboard do now include better quality solid capacitors which are much more tolerant).
Proper load testing is the only way to find out if a PSU can really handle its rated power, so its certainly nice when a review gives all the data. Sure any and all brands of PSU can and will fail, but its nice when advising friends and/or clients/colleagues about hardware to know that the product really is decent.
