I know all the current studies show that you should keep your beer in primary longer as there is no negative effect's. I left one of my standard APA's in primary for 3 weeks before when I would usually only give it 7-8 days and I noticed a big difference in flavor. It was way more "yeastie" so to speak and different from when I just let it ferment and clean out in primary then transfer and age in secondary. I know LOTS of guys will disagree with me but this was based on a beer that I brew often and tried to do the primary only technique instead of my normal routine. I will not do that again. Some guys like that taste and most don't even know what it would taste like doing it with a primary then secondary as they only use a primary. I think it really breaks down to personal preference, beer style and how much you want to work. I might try it again but based on what I tasted last time doing this, I prefer to do a secondary as of now. To each their own and as long as its good beer, who cares.