Ah, a pair of patient teachers. Thanks westfw and florinc. Now I get it.
To Grumpy_Mike: What my "rubbish coworkers" were ignorant of was the extent to which the Arduino environment is optimized to make it easy to program as opposed to using a micro efficiently.
And as inventor of the Roomba (the non-code part of course), I know that the person who wrote all the original code for Roomba did everything he could fit it on the micro and make it run as efficiently as possible to keep up with the inputs and outputs. That's what got my thinking about this in the first place.