何时以及为何不在C＃5.0 ASP.NET MVC 4或Web API中使用基于任务的异步编程

I'm trying to get my head around asynchronous programming in C# 5 and the question that I keep asking myself is why not use it all the time (for ASP.NET MVC 4 or Web API), like the node.js guys are doing? I'm trying to get my head around asynchronous p