There is a ton of threads on this..nearly daily. You will find the reason's and justification and anecdotal info on it by simply doing a search for "Long Primary" or "No Secondary" or even "Yeast clean up after itself."
This has been done to death including the science, the stories of why we do so, quotes like the palmer one below, and even the counter arguments.
You will not have any trouble searching through the discussions. In fact you can start by looking through the similar threads box below...You'll prolly find the info right there.
But I will leave you the quote from John Palmer on the subject.
From How To Brew;
Leaving an ale beer in the primary fermentor for a total of 2-3 weeks (instead of just the one week most canned kits recommend), will provide time for the conditioning reactions and improve the beer. This extra time will also let more sediment settle out before bottling, resulting in a clearer beer and easier pouring. And, three weeks in the primary fermentor is usually not enough time for off-flavors to occur.