the screen optional point seems moot, since it would cease to be an iMac if there were no integration. iMac is the general/basic consumer model, (no offense meant here) designed to fit the uses and budgets of those who are not professionals and do not know a whole lot about the different aspects of computers. However, due to the design of the system and hardware, the iMac can be used as a power computer, the integration of features and limited modification/replacement abilities place it in the category of general consumer. Therefore, offering a computer with no screen would make sense to all of us, it would not make sense to Apple ( or should I say: would not make "Cents" to apple).
Again, just an observation, not an expert.