The Diamondbacks seriously need to upgrade their bench.  Right now, the team's usual starting lineup is as follows:

C Miguel Montero
1B Brandon Allen
2B Kelly Johnson
SS Willie Bloomquist
3B Ryan Roberts
LF Gerardo Parra
CF Chris Young
RF Justin Upton

This leaves the following players on the bench:

SS/3B Cody Ransom (R) .091 .167 .091 11 0 1 0 0 0 1 3
3B Sean Burroughs (L) .218 .214 .236 55 3 12 0 4 0 0 10
OF Collin Cowgill (R ) .000 .000 .000 8 0 0 0 0 1 0 4
1B/LF Xavier Nady (R ) .262 .303 .385 187 24 49 4 32 2 10 43
C Henry Blanco (R ) .222 .279 .444 63 5 14 4 5 0 5 15

Just a couple of weeks ago, the Diamondbacks' bench included Wily Mo Pena, Geoff Blum, as well as Bloomquist. At least that group had a legitimate power threat, an experienced pinch-hitter, and a versatile fielder who can run. But the current group doesn't really have any of that.  The only left-handed bat on the bench is Burroughs, who has not drawn a single walk in 55 AB, and only has one extra-base hit (a double) all year. That is unacceptable for any hitter, but especially one who is only hitting .218 and has struck out in 18% of his Plate Appearances. Burroughs' comeback is a great story, but right now, he should not be on a Major League roster, much less the team's only left-handed pinch-hitting option.

Henry Blanco and Xavier Nady's spots on the bench appear to be safe.  Blanco is fine defensively, and can at least slug an occasional home run in his spot starts. Nady has been so-so, but at least is adequate against LHP (.730 OPS in 2011).

Aside from Burroughs, the two other players on the hot seat are Collin Cowgill and Cody Ransom. Cowgill has only had 8 ABs with the Diamondbacks, and has done nothing worthwhile so far, but I think he is a useful player to have on the bench. Cowgill is a plus-defender in a corner outfielder spot, with an excellent arm, and can even fill in at CF if needed. The team needs a true outfielder besides the starting three of Upton, Young, and Parra. Players like Roberts, Bloomquist, Allen, and Nady can play LF occasionally, but none of them are very good defensively, and none are capable of helping in CF or even RF. Cowgill is also a very good baserunner, and hopefully will start to show some of the offensive skills he displayed in AAA (.354/.430/.554, 30 SB/3CS).  As for Cody Ransom, he also had excellent numbers in AAA Reno (.331/.412/.657), but he also has a long history of mediocrity at the Major League level. He has spent parts of the last ten seasons in the Majors, and has a career line of .222/.306/.381. On top of that, he is not a very good defender at SS or 3B, the two positions where the team needs him to play. 

So what are the options for the Diamondbacks? I think the preferred bench for this team would have been Blanco, Nady, Blum, Cowgill, and Pena. That group has some power, speed, and defensive ability. But Pena is now a Mariner, and Blum is out for a few more weeks with a broken pinky. I would definitely drop Sean Burroughs from the current roster, and maybe Cody Ransom too. The problem is that there just aren't any good replacements in AAA right now. I do think Tony Abreu makes some sense, because he is a decent defender at 2B, SS, and 3B, and because he is a switch hitter. Unfortunately, he was terrible in 2010, but he was OK in his previous stints. The other problem with Abreu is that he also hits better as a RHB, so he won't be as useful as a PH against LHP.  Still, I think the team needs a bench player who can play multiple positions, and while Blum is out, Abreu may be a better fit than Ransom. Who else is in AAA?  Juan Miranda is back in Reno, and is a better hitter than Burroughs. He would certainly be a more useful left-handed pinch hitter. Cole Gillespie had a shot last year, but as a right-handed corner outfielder, isn't really a good fit. A veteran like Andy Tracy is unlikely to provide much, and infielder Mark Hallberg probably isn't ready offensively. There are many good players at the AA level (Goldschmidt, Wheeler, Krauss, Pollock, Eaton, etc), but they all need to be playing every day and not sitting on a Major League bench. So I think the best choice would be to add Abreu and Miranda to the bench in place of Burroughs and Ransom. It's not a huge difference, but I think it would slightly upgrade the team both offensively and defensively.