There are people that buy books based on the cover alone, and you'd better give them what they paid for based on that cover (of course the cover comes after the book in publishing, but that's besides the point). So, whether or not we "should", we obviously do judge a book by its cover. In fact, oftentimes its the first impression of a novel.
So it is with people, too. Most of us are a lot more simple than we seem. We enjoy simple things, often popular things. We all hate that stupid dip in the road that looks invisible every time, etc. We all function on the same fundamental levels as the next person on the street. Sure, we may not agree with them on many points, but even if one train turns left and one train turns right, are they not still trains operating on the same track? Would it not be likely that they are both the same type of train?
So if that person in your class that seems like a terrible person that treats people poorly or never shows up to class on time is, using Occam's Razor, most definitely exactly that: a terrible person. But if you get to know him you may be able to start understand his mentality. Of course you probably still wouldn't agree with it, especially given the fact that judgments are difficult to change once they cement, but perhaps things will make more sense.
Obviously we all judge. It's impossible not to. We use judgments based on depth perception, decision making, and forward planning based on judgments. So how could we possibly turn off that mechanism in regards to people? And even if we could, I don't think it would be a good idea. Personal judgment helps keep us safe and make correct choices for our lives.
I personally think that all of this is pretty obvious, but many people don't go about our daily lives pondering things like this. Many people don't look at more than one side of a subject. I wish it weren't the case, because I believe that forming a three dimensional grasp on every concept we encounter lends us to a healthy, more educated lifestyle, but a lot of us are so narrow-minded that that sort of mindset wouldn't even register in their head. At least not at first. I admit even I can be short sighted sometimes. I don't know if we are born like that or if it's simply not something we are taught, but I think it should stop.
We should all judge our books by their covers, but we should still read some of them to gain a better understanding of the world.