All these things come at a dollar price, so one merely adresses value for each. I have an iMac and a macbook air, all for productivity purposes. I added an iPad mini because the price was reasonable, and it is an upgrade from the original iPad. I don't really see value for my needs in the next cycle of the iPads, so like the iPhone cycle, I'll upgrade to a 5s from my ip4 when there is no line. My air provides something of a "bridge" to the next cycle.
I rather think this is part of AAPL's strategy with a product line, and incremental refresh. When customers see value, they purchase. I really don't see that with the competition. I saw a surface last night. It seems like a device that can't decide what it is supposed to be for. If it's a tablet, it seems kinda...crappy. If it's a laptop...it seems kind crappy. Value to me is two device taylored for the need they address. And they all...work together nicely. That's value to me.