There is a substantial issue with this pattern for things like files -- the operating system imposes a max limit on the number of file descriptors you can have open.

Defer gives no guarantees about when that thing is going to run -- just that it is after the function returns. Because of this, you can get erratic behavior if you open and defer closing a lot; essentially openFile just stops working at some point.

This can happen with any limited resource and isn't theoretical; I had a friend who ran into this in long running process with a lot of IO.