In Variable watchers do not move in presentation mode - #4 by jens you give the rationale for disabling dragging as "that is the official intended behavior for variable watchers, in part so you can use the sliders without accidentally dragging the watchers around" - I imagine that accidentally right clicking a watcher is much less likely than accidentally dragging one.
Here's my use case. I want my program to ask the user to drop a file on the project and then after I've processed the data to save the modified file. The first part works fine in presentation mode while I don't know another way to have the user save the modified data.