Why async by default? Because defaults should encourage best practices. It can handle C10K problem out of the box. With libraries like Q, the code can actually be rather easy to follow and maintain.
One of the biggest reasons why a multi-user server should be coded in async-by-default style is as follows: often, you want to obtain objects and not care about how they are obtained. In procedural languages, this isn't possible. Consider the following challenges:
1) Query sent to 10 different shards, wait until they all return, combine the results in the app, and then use them. In a synchronous language, you HAVE TO go to one shard, then another, then another, and the time is 10x longer. In an async style, it's only as long as the longest query!
2) Some of the objects may have already been cached, whereas others need to be obtained. In addition, the ones that need to be obtained shouldn't be requested more than once at the same time, but callbacks should be placed on a waiting list.
3) Various parts of your code (e.g. from different requests) might want to request the same object. Not only that, but you might want to batch your requests. That is to say, wait 10 milliseconds and see if any other (unrelated) parts of your code also want to grab some objects from the same DB table. Suddenly you are issuing a lot less queries because you can build middleware like this easily in JS. Not so in PHP for example - I would know. A lot is wasted there.
4) Let's consider #1 some more. The queries may go to other HTTP endpoints - do you really want to wait that long for I/O? Furthermore, even if they are going to local MySQL shards, your site becomes slower with an extra factor of O(log n) with the number of users. Kind of like when you use a relational database instead of a graph database for one-degree-away lookups. It could have been O(1) but instead you introduced another factor of O(log n). Not terrible, but certainly gives you a 5-20x cost down the line.