They're not really comparable to Bluetooth receivers. The killer feature is synchronized multi-room audio without paying exorbitant prices for proprietary speakers (Sonos I'm looking at you...). I have them hooked up to powered speakers (PC 2.1 style) in the bedroom and bathroom, a mini-stereo on the deck, a ghetto blaster in my workshop and my media room AV amplifier. For parties it's fantastic to have seamless audio around the whole house.
I have to wonder why Google did this. It seems like a strategic move to push people towards their "smart" speakers. Personally I really don't want something with a microphone in it, just good quality sound at a good price. Very sad.
By the way, it's easy to cast audio only from YouTube or any other app - just go to the Google Home app and cast your phone's audio to the device or group.
My only gripe is that sometimes you can keep your phone in your pocket and use the volume buttons to adjust the casted volume, and sometimes it doesn't work. Might be an iOS thing though.
Also purchasing a gift when it’s price is slashed due to it probably being discontinued is.. risky to say the least.
That said, nothing stops another company from stepping in and taking a license for a smart speaker without neither the microphone nor the speaker, in a tiny form factor...
There's already a bunch of audio-only Cast-enabled products without a microphone. There's no need to license google assistant just to get cast. Speakers & receivers with cast built-in has been a thing for a few years now. The dongle was just an upgrade path while the broader ecosystem built up.