Encuetros are an attempt to optimize the quality of the dance partners. And wherever you make an optimization, you give less weight to other aspects and have to live with the consequences.
I was recently with my girlfriend, and more or less by chance a couple of friends came along. And the couple told us, beaming with joy, that they had registered for an encuentro for the first time and asked what I knew about it. Well, I had to explain to them that I am also registered there, but not with my girlfriend, who had just cooked a loving and good meal for all of us. It takes a certain clarity and transparency in the relationship to set off together in a positive mood for the evening milonga.
Of course it is also possible to optimize regarding a "fair" criterion. This is even possible with the simplest means for each individual tanda. Everyone who wants to dance lines up in two rows for leaders and followers, and a coin toss decides in which direction the rows are brought together. Everyone would feel equally included; strangers, beginners, introverts, wallflowers, creeps. (Okay, okay, nowadays two more rows for double rolers if you like.)
The only question is whether a criterion is to put weight on, whether it is really wanted. As a hint: in a leisure activity like tango dancing, we will observe in reality only that what is really wanted.