Waki-Gatame is one of the nastiest, vicious and unsportsmanlike techniques used in judo. It's illegal of course but that doesn't stop some players from using it. (Tom Hayton has pointed out that it's illegal when done with a throw).
What's worse is sometimes they actually get away with it. In one case, Cho of South Korea actually won a world championships using this illegal technique. In another, more recent case, Mukanov of Kazakhstan nearly won the final of a world championship using this technique multiple times in the same match. He never got penalized for it but in the end he got thrown for Ippon.
Currently, the penalty for Waki-Gatame is Hansoku-Make but I don't think that's severe enough. A player should be suspended for some time if he or she tries Waki-Gatame because it is that dangerous. As Daniel Lascau put it (in the clip below), when you do this technique, you're no longer player sports but trying to break someone's arm. And unlike an armlock, there is no chance for them to submit.