Asynchronous scales better — for sure. But there are cases for synchronous communication. This is like in traditional off-line work — you mostly work alone, sometimes in pair and from time to time you make a meeting.

An ideal communication tool would be one that integrates all the possible modes of communication. There are more dimensions than just synchronous/asynchronous — for example article based (like in a wiki) / short notes based (like in forums). Many years ago I have made a list of them — but now I cannot find it :(