For those talking about clarity, it really depends on how well you can rack with no sediment transfer.
When I first started, I did the auto siphon held by hand and always got a little trub. Then switching to the dual port carboy cap that holds the cane relatively still and at the same height, the amount of trub transferred out of primary was greatly reduced. Now I ferment in kegs and use a brewers hardware sanke fermenter that does not move at all. I just racked a jalapeno ale that sat in primary for a month and a half, and it was crystal clear straight out of primary.
If you're not making secondary additions, don't need to free up your fermenter, and have a good system for sediment free racking, I don't see a need for secondary.