Do not look at these weeds only from the nitrogen fixing point of view.  They can severely deplete nutrients from a good fertile soil.

Dr. Gosh I will appreciate if you could add some valid references losses  in nutrients vis a vis these species.

My opinion (experience). Depending of intensity and moment of interaction between the weeds and the crop, weeds increase lodging in cereals. I worked with wheat and barley in south Brazil- if the competition/interaction occurs in beggining of crop cycle, the crop stiolate and present a slow developement of roots (because the priority is aerial part), favouring future lodging. Othervise, if high-density vetches grow at late, they use the cereal as support, and literally can bring down the plants because of their weight. Here we crop barley and wheat with 17cm between rows, and this problems I cited can occur on high weed densities.