I can imagine you would want to play the same music through the house when you have a big party. Other than that I don’t see much of need. You can simply have a sound system in each room so each family member can its own music , or carry a movable device with you.
note: one of the reasons i saw a lot of this was the company i worked for acquired a structured wiring company (system in a box) that did lots of home/office AV work
The only additional cost was the slight increase in wire length due to the minor path diversion.
Its not just a holiday thing, its a friday/saturday night thing, or a Saturday/Sunday day thing, or a work from home thing, if I want some instrumental stuff, and its also going outside.
I had a receiver in my old place in my living room, and we were gifted a sonos:1, and the sheer convenience of sonos made us not use our expensive home theater set up in lieu of the $199 bookshelf speaker- and it sounded pretty darn good as well. It integrates with my music library, Pandora, sonos, and a bunch of other stuff I don't use.
I now have Sonos amps that connect to speakers in the ceiling. I am not quite as F'ed as others, but those amps are $600 a piece to upgrade, and I have 6 in the house. I really enjoy it, and its better and cheaper than a receiver in each room, and sounds great. Another benefit to Sonos is that its hidden away- the speakers are in the walls, the amps run down to a central cabinet, and in a townhouse that is not small, but still every sq ft counts, this is a big win.
It may not be for you, but I really enjoy it. This was not a pleasant email to read- especially because I don't really want anything more out of the system, particularly the amps- they need to accept a stream and play it, that's it. I bet this has to do with alexa/google assistant/siri integration that is being forced down our throats in all new devices.
The idea of a movable source device makes sense, and I suspect a very common use pattern will be playing something over airplay/airplay2 from a iphone. The zone splitting just makes it possible to have more granular control over where that sounds is going.
I debated a lot about rather to install speakers at all, but I do want to be able to have some housewide paging/notification, and I would rather have each room have stereo speakers in the ceiling ( with a corresponding sub for some locations) then rely on a desk or table held speaker.
Also, the cost of doing in wall is pretty cheap now if you are building new.
Keep in mind it is possible in 3 years I figure out I did it all wrong, but that is the fun of home building. ;)