Displaying corrupt data in the UI can be just as bad as saving it to disk, if the user actually reads and uses the data you display (and if not, why are you displaying it?).

And since the UI almost certainly shares data with the rest of the program, bugs in the UI can corrupt the underlying data.

Separation of concerns is great, but if you really want different parts of your system to have different reliability constraints, then you need to truly separate them, for example by running them in different processes. Running them in the same process but having some of the code be more lax about state because it's less important is just begging for horrible bugs.