Now, I get it. Why wouldn't the Diamondbacks want a hitter who has posted these OBP's in the past four seasons: .408, .428, .415 and .426?
The question isn't so much why would the Diamondbacks want Nick Johnson, but rather, is that the best way for that team to spend their money with Conor Jackson healthy again?
Arizona has numerous holes on their roster. Their current everyday second baseman has a career OPS of .646 against right-handers. Their rotation consists of Dan Haren, Max Scherzer, the rehabbing Brandon Webb, and two other question marks. The back of their bullpen consists of Chad Qualls, Juan Gutierrez and Aaron Heilman, which shouldn't exactly excite anyone.
The Diamondbacks already have their outfield set with Justin Upton, Gerardo Parra and Chris Young, and that leaves first base wide open for former top prospect Conor Jackson. You know, the same Conor Jackson that posted a .363 wOBA in everyday playing time during 2007-2008 and a 3.5 WAR in 2008? He missed most of 2009 with Valley Fever, but apparently he looked good in the Arizona Fall League and I don't see why Jackson wouldn't deserve everyday playing time if he's back at 100% for 2010.
I get that the Diamondbacks need to improve and first base is essentially the only spot in the lineup where they could get it. But what about hoping for continued development from Upton, Parra, Stephen Drew and Miguel Montero? Repeat season from Mark Reynolds and Ryan Roberts? Quality returns from Young and Jackson?
Given that Arizona's offense is flush with talent and offensive upside, albeit with large risk attached as well, I think that the Diamondbacks would be better off trying to address the holes in their pitching staff with the limited payroll flexibility that they have, pursuing pitchers that can eat innings rather than an injury-prone first baseman with a similar skill set to a younger first baseman that they already have.