UI blocking can make sense in some situations; otherwise things "happen" suddenly that are unexpected.

I'm making a simple plugin for Gimp that sends the active layer to a model with a prompt; on Macs by default the UI waits for the request to come back or timeout; on PCs it doesn't, so the modified layer appears unexpectedly. The Mac experience is better IMHO and I will replicate it in the PC version rather than the other way around.