1) Should you learn to use Emacs? Probably. 2) Should you try to switch all your development over to Emacs? No. I'll discuss each of these questions in sequence.
1) In this question, Emacs could be replaced with Vim or any other cross-platform editor. I wish to emphasize the fact that it is cross-platform. This, for me, is the primary benefit of being fluent with Emacs (or Vim). No matter what computer I'm on, it will either already have Emacs on it so that I jump right into some development or I'll be able to easily download it in a few minutes. In my university programming labs, rather than trying to use whatever IDEs the computer I was on happened to have, I just opened the terminal and used Emacs. Since I was already somewhat fluent with Emacs (although by no means an expert), I was able to get the work done just fine without having to fight with a different environment.
Further, there are some languages for which decent IDEs simply do not exist. For example, every time I try to find a decent IDE on OS X for Haskell, I always end up falling back on Emacs. If I wasn't fluent with Emacs, I really don't know how I'd write Haskell code.
Finally, using an IDE for very small projects (and especially just simple one file programs) feels clunky and counter-productive; it's nice to have a simple text editor for such tasks (although I'm sure some would argue as to how simple Emacs really is). But this isn't necessarily an argument for Emacs specifically; on OS X, for example, TextMate is an awesome editor for these tasks.
2) Emacs is powerful and wonderfully extensible, but in my experience it is not a substitute for an IDE. When starting a new project in my spare time, one of the first things I think about is whether there's a good IDE for my language and technologies of choice. On OS X, for example, I'll usually look first to see if there's a good way to use Xcode for my project. On Windows, I'll see if I can use Visual Studio. If that fails, I'll look at IDEs such as Eclipse and NetBeans. Only after that do I consider doing serious development using a text editor like Emacs.
I hope this helps!
