Expense aside, the interconnects (such as USB) made external storage much more prohibitive in the past. Firewire helped for sure, but USB3 and Thunderbolt now make external drive access reasonable. I imagine in Apple's mind, they believed that they could reasonably drop the HD and ODD for flash which ought to be more durable, reliable, and drop the weight of the laptop. Thin is one thing, but the weight was the biggest determining factor in my opinion. Dropping the weight of the 15 in rMBP was a big deal for people on the go. I understand your point. Having plenty of storage on a laptop is important for professionals (or anyone with lots of stuff on their drive) who take their work with them. Carrying around an extra drive is an annoyance. I'm sure that they've collected usage statistics on people's laptops though. I wouldn't even be surprised if they had the techs log the space used in macs brought in for repair. If a vast majority of people are only using some 50-75 Gb max, then the shift to SSD at 128/256 Gb doesn't seem all that unreasonable. The hard part is when you don't fit in that category of course...
Thanks for the reply, you make some really god points I hadn't thought of, about the weight of the device and the usage scenarios apple must have tracked.