Judges ought to set time limits for closing arguments. Not doing so is inhumane, in my opinion. Like Scrbblchick said, if you've already been through a week or two, or three of testimony, sitting for another couple of hours and listening to a couple of attorneys wrangle with each other is not just boring; it's exhausting.
A good, thoughtful closing argument can really help a case, though. If an attorney carefully walks through the evidence and shows why or why not it's believable, reliable, etc., and pulls everything together for a jury, it can make all the difference in the world in how the jury considers the case.