They shouldn’t be for the whole event
Exactly.
but at one of the milongas I really like the DJ will announce “this is a mixer tanda, you dance a song with someone you have never danced with before!”. And it’s great, everyone in the room asks a new person to dance and gets one dance in. Then the DJ says “switch persons and dance with someone else completely new”. Usually does it for three songs.
This is cool, and I have experienced something very similar. There's nothing wrong with it as a small piece of the evening's entertainment. It's up there with a birthday vals, a tanda tropical, a short demo, or another 10-minute pause in the usual structure . . . I'm sure complete and utter pills don't like it, but they can always use that time to get a drink, change their shirt, or whatever instead of being part of the community.
The only thing I think is important is for "Cambio!" to come
between tangos in the tanda, not
within a tango. I've experienced the latter, and it's less fun than it is chaotic, with people standing around mid-tango looking for partners while other people try to avoid collisions. Between tangos, it works better, especially if the DJ injects a little 5-second pause.
The one I liked the most was probably the mishmash one where they came up with a ton of silly ideas to encourage people to dance with each other.
At the place I used to work, the organizer would have a "leader" hat and a "follower" hat, and people who wanted to play could write their names on little pieces of paper and put them into the appropriate hat. At some point later on, she would draw from the less-full hat and then the other hat to create couples for one mixer tanda (there would, of course, be names left in the fuller hat, and those people and people who chose not to participate could dance as they could arrange it). The chosen tanda was always of whimsical, festive, or goofy tangos (usually ones with singers and laughter), not dramatic, romantic, or sad ones.
Anyway, I liked it. I ended up dancing with a couple of people I wouldn't normally have danced with, and I discovered that some of the tangueras I avoided as "serious" had avoided me as "serious," when in fact neither of us was serious, and after that we danced together more often.