He’s a defensive guy. Here’s the stuff on him from his wikipedia page (the Jets Mangini page has been taken down):

Prior to coaching the New York Jets, Mangini worked for the New England Patriots under Head Coach Bill Belichick, first as their defensive backs coach (2000–2004), and later as their defensive coordinator (2005). Before that Belichick gave him a job as an assistant in the Cleveland Browns organization (1995). Mangini was part of the Baltimore Ravens’ inaugural staff in 1996, where he was a quality control/offensive assistant coach. From 1997–1999, he worked with the Jets as a defensive assistant/quality control coach. That was also when Belichick was assistant head coach/defensive coordinator.

Looks like he was offensive assistant early on but became a defensive guy eventually.

6) he was the QB coach when martz was the OC in st. louis…again, continuity.
7) he made jon kitna a good qb in CIN…another martzian quality
8) justin smith and ahmad brooks can tell him how great it is to be in san fran

There have been some inconsistencies, but the offense is much improved over last year. One reason for keeping him is the need for some kind of continuity. Over the last six seasons, the 49ers have had six offensive coordinators. Given the complexities of NFL offenses and learning new systems, keeping Martz would allow the offense to grow at a normal pace. Martz runs an especially complicated offense, so bringing in somebody new not only means learning a new offense, but trying to forget all the gibberish in your head from Martz’s offense.

Of course there are also reasons for dumping Martz. One is that the Martz offense is so combustible that it doesn’t seem like a long-term solution. Also, the he was strictly a Nolan hire. McCloughan and York did not want him because they felt the 49ers personnel were not suited for that style of offense.

Me personally? I like the contrasting personalities and I think Singletary has shown he can keep Martz in line. At the same time, an improved year in 2009 and Martz could be off for another head coaching job. It’s a big pain in the butt is what it is. I think ideally, I wouldn’t mind finding a young position coach that would be our OC for more than just a couple seasons. I think that create the most long-term stability.

Maybe. One thing we know for sure is, like Fooch said, the offense improved drastically under Martz. We also know what Martz brings to the table as an OC and the same can’t be said for some other candinates who haven’t been OC before. If I’m Singletary I would have probably stuck with the known commodity here.