It is of course difficult to judge objectively whether a discussion is productive, but there is some literature on the subject.
A 2010 review  on classroom participation in college says about graded participation:
Course policies on participation as set by the instructor also impact student participation. Berdine (1986) and Smith (1992) suggested that whether or not students participate depends on how much their participation counts toward their final grades. The “pearls of wisdom” approach where students record their participation each day to count toward their end of semester grades was found to be effective in increasing participation in the assessed course and reported to increase participation in other courses (Junn, 1994). Fassinger (1995a, 2000) suggested that students should earn extra credit rather than counting participation as part of a student's grade, and Boniecki and Moore (2003) and Smith (1992) found that rewarding students with extra credit did increase participation.
Mandatory participation (Dallimore, Hertenstein, & Platt, 2004) and calling on students, even when they have not volunteered (Auster & MacRone, 1994; Dallimore et al., 2004) can both be effective practices for encouraging participation. However, Moguel (2004) noted mixed perceptions of cold-calling, and Karp and Yoels (1976) noted that this happens in only about 10% of classroom participation.
The paper by Dallimore, Hertenstein, & Platt  mentioned above uses a student survey, and asks students what they thought was effective at improving classroom discussion:
A questionnaire was administered to students on the last day of the course. We asked students to respond to questions regarding what professors do or say that: (a) increases student participation and (b) either increases or decreases the effectiveness of the discussion.
Some students cited graded participation as increasing student participation:
(This is part of "Table 1 Student Responses Regarding Quality of Participation and Effective Discussion Factors")
However, they used cold-calling alongside grading participation, which students seemed to find important:
Notable in Table 1 is the fact that graded and required participation is a major category that emerged for both quality and effectiveness. Respondents repeatedly identified the importance of graded participation, suggesting that instructors should "make it a significant part of our grade." When asked what a professor says or does to increase the quality of student participation, grading and requiring participation were regularly mentioned (including responses which would be considered cold calling according to our definition). For example, one student observed, "the fact that professors call on most students to answer a question increases my incentive to prepare [thus enhancing discussion quality]."
A more recent study , also using a student survey, found that in a survey before the course,
43% of students surveyed indicate they participate more when participation is graded.
After the course (which had required, graded participation), a little over 40% of students agreed or strongly agreed with the statement "I participated more in this class because participation was graded":
and roughly 30% of students said they participated more than in other courses:
(Table 6: Participation Comparison: Current Course to Prior Courses)
 Rocca, K.A., 2010. Student participation in the college classroom: An extended multidisciplinary literature review. Communication Education, 59(2), pp.185-213.
 Dallimore, E.J., Hertenstein, J.H. and Platt, M.B., 2004. Classroom participation and discussion effectiveness: Student-generated strategies. Communication Education, 53(1).
 Paff, L.A., 2015. Does Grading Encourage Participation? Evidence & Implications. College Teaching, 63(4), pp.135-145.