This is a really good question and consumers have obviously become infatuated with roof prism binoculars. Porro prisms have several advantages and one glaring disadvantage:
They have better depth perception than the comparable roofies
They have one less prism, light changes direction one less time so they are lighter and easier to make perfect than a comparable roofie.
The best feature, IMHO, is that they can be pretty much any size you want and still be made to work with normal human IP distances.
About 25% of the population cannot use the Swarovski 15x56 SLC because they don't get close enough to fit many eye spacings. With normal size glass, this isn't an issue.
The drawback is that they are, for some reason I don't understand, more difficult to make truly waterproof. I have seen the Super Es used in some pretty nasty weather and they were fine- just don't take well to submerging.
We have fallen in love with roofies and this has killed the porro prism R&D which really sucks!
