The synchronous vs asynchronous communication argument is on par with a “best programming language” debate. The style of communication is just a tool that should fit the need and the tasks at hand. If you are working on a largely engineering driven project with a thorough spec where "heads down" development can be most effective than email/messages between team members probably will probably be effective. However, when developing on a small-sprint, iterative agile team that style of communication would be a death knell to effective communication.
Not to wave my banner in the holy war, but that's just, like, your opinion, man.
The thing I like most about async is that there must necessarily be a record of everything that was said. When appropriate, the record may be retransmitted.
Things like internal wiki entries, message boards, e-mail chains, instant message groups, and issue-tracker tickets are all asynchronous. If you conduct most of your business with ephemeral synchronous conversations, you're going to end up repeating yourself a lot, and there's going to be a larger "hit by a bus" risk. I consider it to be irresponsible business practice.
There's good reason why human development was very slow during oral tradition, and experienced exponential growth after writing.
Some use of synchronous is always appropriate. But using it instead of asynchronous rather than in addition to it is, in my opinion, pure folly. At best, the synchronous communication is what you use to make your asynchronous communication more efficient.
Another huge benefit: non-native speakers. If all your office processes rely on verbal conversation, it's really critical that everyone speaks perfect English. If the communication is async, that constraint can be relaxed slightly, which can give you a great pool of talent other companies are missing out on.
