Whereas solutions like Meet and Zoom are designed primarily (both UX and architecture) around multi-party, supporting 100's (250 for Meet, 500 for Zoom).
But under the hood, limiting the number of users allows some important technical advantages. Duo claims (in its FAQ) to be end-to-end encrypted. I believe Whatapp claims the same. And it wouldn't surprise me, knowing Apple, if their video chats on Facetime did too. This is possible only with a reasonable limit on the number of participants because of the need to negotiate a bandwidth for each stream that works for all viewers, and each stream needs to be independent, not multiplexed into a single live view.
The proper use of large-scale meeting products is in corporate or paid environments where you have some kind of guarantee that your chats aren't being recorded or sold. (At least unless a three letter agency is involved.) Tools that guarantee end-to-end encryption should be used in small group chats.
Your point about UX is a strong one too. There are a affordances that work better for smaller chats, others for larger ones, and it's helpful to be able to specialize.
In banking/financial services, we want guarantees that all chats are being recorded. In lots of other regulated and government fields too.
Because beyond UX, there are a ton of technical tradeoffs involved.
In a 2-person call you want to avoid a server if possible, because it adds latency -- so that's a good first reason. Zoom and Meet would never dream of not using a server.
The optimal architecture of a "small group call" is totally different from a "large group call".
I agree though, this feature seems like a solution in search of a problem. If I'm answering the call it seems more natural to activate both cameras at the same time, and for calls I'm not going to answer the video feed just seems unnecessarily distracting. I'm sure that I have never wished for a feature like this.
Seems like the sort of thing that perverts would take advantage of.
Presumably you would know who you're video calling anyhow prior to calling them, so there's not really a meaningful privacy violation...
The only case I can think of is you calling me to expose yourself or something - to that, I'd say that a privacy option where you can make it so you have to add someone before they can video call you would suffice.
One of the problems with an opt-out setting is that it can become a viable attack vector whenever the application settings reset: "OMG New phone for my 14th birthday" will look like an invitation to some segment of the population.
http://support.google.com/duo/answer/6376115
Google knows about dick pics.
And limiting it to contacts doesn't solve the problem.
Why should my phone be displaying something chosen by someone else unless I explicitly opted to engage with them (in this case by answering a video call)?
Plus, even if I did have contacts with a dick pic sense of humor, I'd much rather see a dick from them after I opt in to the call (so that I make sure I'm in an appropriate setting). Getting one randomly is insane.
Imagine if your husband or wife called you for some phone sex (say you are on a long trip) but you happen to be in the elevator with strangers on the way to your room. Brrring! No thanks.
The "knock" feature is kinda novel and weird (in a good way). When you call someone, they can see your video feed before they answer the call.