This is interesting. I believe 110% that this is true, that without e-mail integration attempts to build online communities behind the firewall will fail. Perhaps this is only true in the law firm environment, but I suspect it is true elsewhere as well.
The challenge is using e-mail as the hook, not the entire chain. Do E-mail notifications and cc'ing an online community on an e-mail allow just enough integration to get people to carry on the conversation in the online community? Trying to integrate entire e-mail chains into an online system does not seem like a path that will work. That means there is no way to keep the conversation from becoming fractured between e-mail and online in the early stages though.
"You need to reduce the barrier to entry to KM whenever you introduce new tools and new processes. Make them as simple as you can, and then simplify them again. Build them into existing work habits, such as email. Don’t expect people to learn new tricks, or make new clicks"
- Knoco stories: No new tricks please! (view on Google Sidewiki)