Here's one example of what often happens...
As soon as the cortina starts, especially if the milonga is very busy, leaders scramble to find their first choice next partners in scattering dancers. Because people are seated somewhat haphazardly and there's no single point of exit from the pista - visibility is tricky, even if the lights are brighter (which is rare). So gentlemen have little choice but to walk up to the table where their intended follower is sitting and simply ask her. Knowing that the milonga is usually set up poorly for the cabeceo, the follower accepts invitations that way, rather than appearing rude and declining an invitation, and "punishing" the leader for asking in the only way he had the opportunity to.
So we usually accept the invitation.
There are times when it would be even more prudent to turn down an invitation that way - such as when repeated cabeceos were *not* returned and then the leader decides to come up directly in front of the follower and ask her to dance. This has happened recently to me with a leader who swings me around like a rag doll when I dance with him and nearly pulls my arm out of its socket to do his big finale tango poses. I avoided his cabeceo for three cortinas - even getting up and leaving my table to avoid it. Finally he just appeared in front of me and asked me.
I should have declined but I didn't. I rewarded the behavior and got a sore shoulder for the trouble. Simply because I thought it would be more rude (and obvious) to turn him down. So in a way, I was punishing the nice gentlemen who might have been trying to make use of the cabeceo by getting up and dancing with a man who had, by the standards of milonga codigos, been rude. I also, by accepting his invitation, rewarded his style of dancing even though it's frequently inappropriate for the social dance floor. One leader said it sends a pretty clear message when they see that happen: Nice guys finish last (or at least sit out more tandas).
So in these situations where at least half of the leaders can't or don't use the cabeceo - is there any way to reward, and thereby encourage, the leaders who do, at least when they're able to? Of course we can be observant and available for the leaders that use it, but usually by the time we think someone *might* be trying to cabeceo us, another leader has already approached and asked.