You don't see how students working in groups.....helps them develop skills for coherent discussion?

I'm not sure which university you've been to but groups typically discourage meaningful discussion because very few people care to put forth the effort to disagree, let alone the effort to revise their argument in a coherent way following a disagreement.

The vast majority of students are there for a degree and career opportunities, or because they don't know what else they should be doing. There are very few that genuinely care to propose a well crafted argument to a peer that likely just wants to go home and work on a project for one of their 4-5 other classes or get ready to go see friends following the class.