Client Projects have a "Close Date" because it's important to always be mindful of how long any sale is taking. Open ended decision making processes that never seem to conclude usually don't. For that reason, it is good to set a realistic time goal for all team members to agree upon.
Close Dates are not as critical for Discussion Projects, but are still considered good practice. Keeping projects open for a finite period of time puts pressure on everyone to express themselves while they still can, and be responsive to others.