Devin Harris is a slightly above average starting point guard
My Top 15 Point Guards
1-5 -- Deron Williams, Chris Paul, Tony Parker, Chauncey Billups, Rajon Rondo
5-10 -- Gilbert Arenas, Steve Nash, Andre Miller, Jason Kidd, Jameer Nelson
10-15 -- Baron Davis, Derrick Rose, Devin Harris, Jose Calderon, and TJ Ford
Devin Harris is better than everyone behind Arenas on your list, and he's better than Arenas at the moment because Arenas hasn't been healthy in a while.
Devin Harris's 2008-2009 stats:
21.3 pts 6.9 assists 1.7 spg
PER: 21.65
That's way above average. Devin Harris might have been deserving of your assessment a year or two ago, but he improved a LOT in the last year. He came very close to getting MIP this year, and was an All-Star unlike the majority of the point guards you have in your top 15, many of whom you put ahead of Harris.
Devin Harris might not play on a great team, but that doesn't mean you can ignore his ability. He may or may not be in the top 5 point guards in the league, but he's definitely top 6-8.
You're also not taking into the account the fact that Harris probably hasn't even reached the peak of his potential yet because he's still relatively young.
Again, Harris is a known quantity: he's going to be a strong starting point guard for years to come. Ricky Rubio might be a star - someday - or he might never be.