Theoretically, in doggie body language, direct eye contact is not polite. It could be a threat or a challenge, or it could just make other dogs uncomfortable. It depends, and I dont' know enough of the particulars beyond what I just said to discourse on it just yet.
To humans, though, eye contact is very important. If somebody avoids eye contact, they don't like you, or they're lying. If they give you direct eye contact, it shows you're important to them, they're paying attention. If they absolutely won't look away, it could be a threat or a challenge (I'm seeing some overlap here).
Let's focus on the "important" thing, though. Sometimes, it's important for our dogs to look at us. It could be so you can give them cues, which could be very significant with regards to safety or with regards to Qualifying on that agility course. It could be so you can take a picture. It could be just to get your dog's attention. Overall, "Look at me" is a pretty handy cue.