I've found that it depends on the song. For the most part, I agree that the most valuable thing which a performer can to to aid communication with an audience is to make as much eye contact with as many people as possible. However, there are songs which require a certain amount more concentration (many of the slower, more intense Stan Rogers tunes, for example;"Song of the Candle", "Second Effort",etc.) and closing one's eyes help toward that end. By the time one gets to the point in a given set that one can seriously think about doing such a song the closed eyes are, as a rule, no longer a barrier because, presumably, one has been maintaining the eye contact up to that point and will continue it afterward.