Modal dialogs suck. As a design feature they are intended to ensure a particular application flow, kind of a critical section for attention - for your personal flow instead of program flow. But the way they work - allowing any number of processes to grab your attention at will - just plain sucks. Modal dialogs always makes me feel like a factory worker in the spirit of Chaplin's Modern Times - why should I have to adjust to the machine like that?
I don't get why it is considered a feature of a user focused operating system to constantly interrupt the user. It is so very rare that the computer is actually on fire, so why is everyone equipped with an fire alarm like this? What would be nice was a user interface with better attention mechanisms, e.g. OS supported "forced focus" or even better "attention subroutines" well done - like the one I described the other day.