Maybe a year ago I would of been very against a feature like this. But now, working on a Haskell code base for quite some time that has grown I have wished for something like this many times. Most often it is when changing intra-module data structures and functions just to try something out, and I do not want to update the rest of the module quite yet. Sure I could comment out the offending code and replace with undefined, but that is essentially what this switch will do for free. So I for one am looking forward to this feature.