In fact I believe that they are hugely important for usability and design, however, modal sections of an application must be chosen wisely. A-Modal design will almost certainly lead to a smaller development effort. It means that the design is primarily functional. That is there is screens and menu options which perform a set of functions. The User is responsible for choosing which order to execute those functions.
Modal design is procedural.
In the same way that Functional Code is more loosely coupled leading to improved re-use and reduced development effort, so this is true of screen design. David Ruble correctly identified this and documented his measure of Window Cohesion in his book which I reviewed. A-Modal designs support different User Roles better and with less development effort. For example, a set of different Role Players within an organisation may need to access the same data but for different purposes and in a different order. An A-Modal design means that one design fits all.