A new version of iPhone OS requires a keynote.
It is Apple's most important software, the operating system of its flagship products. It needs the marketing push of a keynote to get developers working with the new APIs and to get users excited to upgrade their iPhones.
iPhone OS 3.0 is the least showy point-oh release of the Scott Forstall era. Version 1.0 made the iPhone possible, 2.0 added the App Store, 4.0 added multitasking, 5.0 added iMessage and iCloud, and 6.0 added built-in turn-by-turn navigation and Passbook.
iPhone OS 3's two biggest tentpole features was something already announced and later bumped from 2.1 (push notifications) and a feature that was likely on the original development todo list for the iPhone, but was so low priority that it didn't get done until two years later (cut, copy and paste).
With Steve Jobs on medical leave, this could have easily been Scott Forstall running the entire keynote from "Welcome" to "Thanks for coming", and I wouldn't be surprised if he asked for that chance. Given that Tim Cook was in charge during most of 2009, and what happened to Forstall after Cook permanently took over after Jobs' death, it wouldn't be too much of a leap to think that Cook insisted on adding an M.C. to bookend the keynote.
In the end, the M.C. was only needed on stage for about four minutes at the beginning of the show and three minutes at the end. That's not a job for Phil Schiller, who did 90 minutes of speaking at Macworld and was in the on-deck circle getting ready for two hours at WWDC. (Schiller was on hand that day, and participated in the Q&A segment after the keynote.)
The M.C. job fell to Greg Joswiak, a fan favorite and thoroughly excellent speaker, yet someone who must share a kinship with NHL goaltenders who get hired to be the third netminder on a given team. He's neither the starter nor the backup. He's put out on the ice by the coach in less consequential games when one goalie is injured and the other is resting up.