We know that yeast activity can result in positive changes to hop oils.
Adding hops toward the end of fermentation (or during fermentation for
that matter) can result in some positive aroma compounds that carry
through to the finished product. The problem is that hop oils can be
scrubbed out of the beer through fermentation (CO2 evolution) and there
are losses due to hop components (oils and resins) adhering to the yeast
cell and being removed from the beer. In other words, dry hopping in the
presence of yeast has some proven positive flavor effects but negative
extraction effects. Once the yeast has been largely removed from the
beer, there is opportunity for better extraction of raw hop character. So
we do both. We try to take advantage of both possible extraction
opportunities and our belief is that we get a more well rounded hop aroma
with high overall hop impact.
The theoretical justification could be argued and certainly great beers
are made with single dry hop additions. The boys at Lagunitas do a single
hop pellet addition for dry hopping and they get a very high hop impact.
You could compare a Union Jack and a Lagunitas IPA and decide which method
you prefer (of course the hop variety, time on the hops, amount of hops
and temperature of the dry hop all play into the equation.)
There are very few straight answers in brewing.