Jeez, people, the defensive depth of the NHL is not what you think it is. Most of the defensemen playing as top pairing D in the AHL and floating around the 7-8 spots have massive flaws in their games, just like Barker. The ones who don't, move up quickly, a la Tanev. Like most of the flawed, Barker also provides a few pieces of upside. Look at a depth chart and then explain how Cam Barker is a terrible 8th defenseman.
He's average. You could argue slightly above average due to NHL experience, but you could also argue he got that experience due to a longer leash as a high draft pick. So basically, he's an average 8th defenseman. And yes, like most 8th defensemen, he kinda sucks. But that's just what an 8th defenseman does.
EDIT: Some comparables in roughly the 8th D position.
Nate Guenin, Jordan Hendry- ANA
Michael Stone-PHO
Nick Petreck- SJ
Justin Falk, Drew Bagnall- MIN
Steve Montador- CHI
Kent Huskins-DET (Now PHI)
Mitch Bartley-NAS
Jeff Woywitka- STL
Michal Jordan- CAR
Eric Gryba- OTT
Aaron Johnson- BOS
Yannick Weber, Tomas Kaberle (yuck)- MTL
Mike Komisarek- TOR
Peter Harrold- NJ
Thomas Hickey- NYI
Stu Bickel (lol) NYR
Kurtis Foster- PHI
Robert Bortuzzo, Resse- PIT
See? Average. Middle of the pack! Notice that many top contenders have the worst, and thus cheapest. A necessary sacrifice.