Are there any disadvantages of using ARC? Is there a use case for disabling it?
Backwards compatibility is a common reason not to use ARC. If you are deploying your code on earlier versions of iOS or OS X, you can’t use ARC.
Also if you are holding references to Objective-C objects in C++ or C data structures, ARC tends to get in your way. (But remember that ARC can be disabled on particular source code files, so you can just not use ARC on your sneakiest classes.)
But, other than those two cases, ARC is really nice and fast.