Getting off topic, but anyway...
I agree that the requirement to have Spotify Premium is silly - that's when we first signed up to it as only the Premium service works with the Squeezebox Duet.
That said, I totally think it's worth the $13 a month to have this over the free version - ads aside (and I'm assuming the free does have ads? I've never used the free one), the ability to save music off-line is a must-have in my book. When away from home, it's a life-saver - for example, think of all the great kids' music like the Frozen soundtrack for the 49th time you can listen to when travelling in car. But even in the home environment it's worth having, as a single-user account is limited to playback on one device at a time. But with Premium we can get around this just by using off-line music and ensuring wifi/data's off on that device - this way the kids can be listening to their cr@p in one room (TMNT etc) while we have something else playing in another.
Of course, the Spark plans are one option to get it "free" (if only they had roll-over data and minutes they'd be great plans!).
But I totally support the idea of going back to a proper stereo; it's a rare Bluetooth speaker that sounds superior to even a low/mid-range stereo. Just like the move away from HT or stereos to using soundbars, it's much more a convenience factor than looking for a better audio quality. Don't get me wrong - I love our MegaBooms, especially in stereo mode, but I love them for their portability, size and battery power, and appreciate the sound is good relative to these other factors and for the purposes we put them to, but never would they replace one of our stereos. Hence why this Chromecast audio will be useful - my wife's old stereo, which is in my eldest son's bedroom, will be given a new lease of life as he'll be able to stream music directly to it.