Everyone else has said what I wanted to. Apart from:
Frankie was given content to write for the updates and was also tasked with PR at events to explain to journalists about Halo when questions were asked. The most he ever did directly with Halo was edit the H3 terminals. How you consider him the "the best thing that happened to halo series" is beyond my brain.
He even said it to High Impact Halo, himself:
I'm content manager, which really means, spare monkey. That means I do everything from writing the manual, helping marketing with materials, running the website [SJ note: lol, got anything to say, Achronos?] and going on trips to incredible places, to shoveling pimp turds out of Marty's recording booth. Seriously. Pimps are way worse than cart horses.
It bothers me that he somehow clawed his way up to such a prestigious position at Microsoft. I think he obviously had some ideas that weren't being listened to at Bungie, so he decided to take it to the top (the weird direction that Halo's been taking in the novels and what we've seen of H4 says that much). Sneaky if you ask me.
And Shishka worked on miscellaneous content like the Halo 2 Attract video (when you left the game sitting on the menu) and some matchmaking playlists. Not sure what else he did beyond that, but again, I wouldn't say that he was the "core of the Bungie/Halo communtiy".
Regarding them moving on, I'm personally glad that Bungie's not the sort of company that milks their games to death (by choice). They recognize when it's time to move on, and I really respect that.