The debate between different sensor formats is very much like the debate between primes and zooms. While top-quality primes may have higher IQ, allow for a more shallow DOF, and be better suited for low-light photography, they do not zoom. That singular advantage of a zoom trumps all the advantages of a prime for many photographers, and so it is when comparing formats. It is not only a matter of whether one system is "better" than the other in terms of IQ, but at what display dimensions this difference becomes significant. For many, and likely most, it is more often a matter of available lenses, differences in DOF capabilities, and operational convenience, than it is a matter of IQ alone.
The bottom line is that we use a camera to create images. It is important to understand the advantages of any particular system as a whole, both in terms of IQ and operation. The purpose of equivalence is to help evaluate the IQ end of that consideration, and, in conjunction with our individual "quality threshold", make an informed choice as to which system, or systems, best meet our personal needs for the photography that we do.