I was sitting at a bar last weekend when a guy walked up beside me and asked exactly this question. He knew the cost of a shot of Patron was more than that, and he was hoping he could get a smaller portion at a cheaper price. The bartender in me hates things like this. I think all people who come into my bar should only act in ways I see fit that make my job as easy as possible; and questions of that nature are not allowed. It's a common theory among restaurant employees that all humans should be required to work in a restaurant before they are permitted to eat or drink in a restaurant, so they know what is and is not appropriate behavior.
But, the logical part of my brain loves this idea. I like good expensive tequila (which, by the way, Patron is NOT), and I like to nurse and enjoy it with many tiny sips. But I rarely order it, because it bugs me to pay $14-15 dollars for a drink. On the other hand, I'd happily get 1/3 size shots of Chinaco Anejo from time to time for $5.
Or maybe sometimes you only have a few minutes in the bar, but you don't feel like chugging a full beer. Why not order a half draft for $2.50 instead of $5? There are tons of possibilities.