Well like I said, there have been time where I would have wanted to do that.
Let me think of example.
Often I play Poker on my computer. The sound of notification are very important as I multi-table. Often 4-6 tables. But I always do lot of other things at the same time in life, including when I play poker.
Sometime I was grinding some mobile game. Now if I play the game and it has sound and it is linked to Bose, it may take over my computer. And I might want the game music on, but still hear my Poker table notification (I'm good at multitasking, but I'm also good to be distracted and forget, so notifications are important to get back my focus, especially if I handle dozen of things at the same time, I can just concentrate on one thing).
That is one example.
Similar can happen too if someone call me on my "real phone" and I do something else on computer, like playing Poker, that require me to react. I like to use my Bose QC35 with phone.
It is just like I listen to music too at the same time.
Sometime you want multiple audio-source active... and... well it happen that sometime I would want my two devices to be connected and output sound in my headphone.
Now the question is... they don't support it because like your reaction, they think people are not gonna need that, blah blah, will complicate the software, or they don't support it because of limitation. Like my Bose QC35... why only 2 devices, why not 3-4-5 at the same time, etc. There are sometime reason, but sometime not much valid reason except someone think nobody is gonna need it and they try to over-simplify something. (I'm a software developer among other things and I see this often and it sometime lead to problem too... as much as I like to simplify, sometime the most power and flexible solution is the most elegant and simple!)
It is just a matter if it can be supported or not.
(There might be other solution for my wish/need, like redirecting the sounds of my phone to my computer if there is solution to do that, etc. and so I will listen all in my computer... but that would be more of a kind of work around compared to a direct support of sound from two devices right into my headphone)