One advantage of adding hops to primary is the yeast's ability to use the oxygen that will be added with the hops. This will promote a longer lasting beer.
Sierra Nevada released a paper recently that found that unique flavor compounds can be created if yeast is actively fermenting while dry hops are added. These flavor compounds were not found by any other process. I can't find a link but it was mentioned on the brewing network.
I dry hop in the primary. I usually wait about 3 weeks and make sure I have hit my FG before adding them though. I feel that I have great results. But everyone has different results. Try it, see how you like it. Or don't.
From what I have read people feel that CO2 being produced will drive off the hop aromas when dry hopping. But long primaries lead to beer that many folks believe is much more clear than if they were to secondary. I wait for fermentation to complete and for the yeast to clean up after themselves abit before I dry hop. They finish cleaning, the dry hops do their business. And I get clear beer that stinks like delicious hops.