Pan, I don't know who's talking out their asses, but a theory, if it's a good theory, is based on experience, not hearsay. But I wouldn't be surprised if the bullshit factor exceeds the experience factor here.
I totally agree with the exception of fishing larger iron hooks to a certain extent can put you slightly more below the water column. You can also use an intermediate poly and obtain the same water level without swimming a larger fly. In my experience, the takes with an intermediate and dry are very similar as well. This is off course dependent on the type of water you are fishing.