Ha! That is great. I actually like that effect. I wonder why.
Actually, this small band of light across the eyes isn't just a Kirk/Shatner technique. We see the technique other times, too. For example, we see the same technique used on Charlie Evans/Robert Walker Jr. in "Charlie X." Actually, it's a tense scene between Kirk and Charlie--a battle of wills--and as the camera cuts back and forth between the two of them, you can see that they
both have bands of light across their eyes--so that the audience will focus on the intense eyes of the two actors as they stare each other down.