Not sure I agree with the FSP aspect, but you certainly find Seasonic, at least in design, in most quality units. With Corsair you're generally going to see Seasonic and Channel Well based units. With Antec it's usually Seasonic and Delta. With XFX it's also Seasonic. Right now I believe OCZ is using Great Wall and HighPower for a lot of their units. Their PC Power & Cooling units tend to be Seasonic and Super Flower.
The common factor here is obviously Seasonic, which isn't surprising, as they've been setting the standard for quality power supply units for some time now. FSP does have some quality units, along with some horribly shotty ones, but I believe your assessment of them in the market place is exaggerated.
As a side note, if you want to get a good idea of who actually makes the PSU you're considering purchasing, what kind of quality it has, and what platform it's based on, this is a good place to start:
http://www.realhardtechx.com/index_archivos/Page541.htm
As for the original question at hand, I would say the biggest advantage is the warranty coverage. With a shelf bought system you get a 1 year warranty and have to pay a significant premium for an ESP. The warranty coverage on most individual retail parts is usually 3 years or more.