I think the point of the argument against is that the air moves so fast that the moisture removal is ineffective.
One big thing to understand is that "drying" air is not about removing condensed water (like what comes out when one vent the traps in a compressor), but it means removing the water vapor that has not condensed out into liquid. It is important to do this on air that comes out of a compressor, before it goes into a reservoir, because that air charge coming out of a compressor is always going to be hotter than ambient temperature - thus it will carry water vapor in it that will condense out into liquid water
after it cools down to ambient. Just trapping the liquid water is not enough.
That leads to the reason for why drying after compression is easier than before for most compressor situations - the act of compression will drive well over 90% (potentially as much as 99%) of the water vapor to condense into liquid water, leaving only a few percent of the water vapor that was in the original air charge to be dried by a typical desiccant filter. It's not that drying before compression is ineffective - it is that it takes a lot more desiccant to do it right.
@F6Hawk is correct in his approach - once the water vapor is removed, it is gone. One can fully "dry" the air we compress before compression just as effectively as after compression provided it is done correctly. One can choose the way they want to go, but some active desiccant drying is approriate either way - just be sure to do it correctly (mostly meaning to achieve fully dry air, safely).
There are tradeoffs through. Dry it before compression and one has to use a lot of desiccant in an appropriately sized filter to insure the air is fully dried, and it will likely need frequent regeneration or replacement. Dry it after compression and much less desiccant is needed (and thus the filter can be smaller), but the filter housing has to withstand the full pressure involved, and the compressed air that fills the filter (and the time and energy to compress it) is lost as it does not make it into the reservoir.
Personally, I use a trusty old Shoebox compressor, and since that is fed by air from a shop compressor I dry it after the shop compressor but before the Shoebox - the first stage of compression in the shop unit removes almost 90% of the water via compression induced condensation, leaving only about 10% of it for the desiccant before the Shoebox to handle. I do no "drying" after compression because the air is already sufficiently dry leaving nothing further to be gained with another step.
