OK that's a provocative title intended to get your attention. But the reality is that Walker isn't really any worse than Capps--and that's not an endorsement of Walker. It's just that folks in NatsTown are vastly overrating Capps (and don't even get me started on Brian Bruney).
To the numbers. There are a lot of fancy pitching stats out there, but the three most important pitcher stats when measuring repeatable skill are strikeout rate, walk rate, and groundball rate. If you can miss bats, keep the ball on the ground, avoid walks, or better yet all three, you're going to be ok.
Here's how Walker and Capps measure up:
Groundball rate (MLB avg: 42%)
Walker '09: 37.6%
Capps '09: 40.7%
Walker career: 40.7%
Capps career: 36.0%
Walk rate (MLB avg: 9%)
Walker '09: 6.0%
Walker career: 8.9%
Capps '09: 6.8%
Capps career: 4.5%
Strikeout rate (MLB avg: 18%)
Walker '09: 18.0%
Walker career: 18.7%
Capps '09: 18.3%
Capps career: 18.6%
So what does this show us? These guys are both flyball pitchers who get average numbers of strikeouts. Right there, that's a bad combination (lots of balls in play + lots of them in the air = lots of homers). Neither of these guys is a true ace reliever.
The place where Capps may separate himself if he's pitching well is in the walks. When he's on, he's got exceptional control, though, importantly, that was not so much the case in 2009.
The other key for a closer though is how well they pitch under pressure. Both pitchers have a lot of experience in high-leverage situations, and neither has done very well.
Career, Capps has allowed a .232 / .260 / .380 batter line in low-leverage situations, but a .274 / .311 / .458 line in high-leverage situations. Walker meanwhile has allowed a line in .252 / .311 / .409 line low-leverage situations, but a .280 / .361 / .461 line in high-leverage situations.
Again, bottom line, neither of these guys is a closer. But if you aren't excited at the prospect of Tyler Walker closing out games, you shouldn't feel any better about Matt Capps.