Hm - I can understand the reasoning behind the functionality. I think the visual is flawed then - it feels like the button shouldn't change to it's pressed state again when you drag back onto it. It sends a visual signal that I'm ready to be pressed, and then feels like a bug when it doesn't perform it's function. I associate the pressed state with "I'm about to do the thing I'm supposed to do."
Additionally, I wonder if it's contributing to a bug I'm seeing - users will press a button and occasionally have it not do anything, despite the pressed state being shown. At first I thought the collider was shrinking with the UIButtonScale script, but I wrote a separate one that doesn't and I still see the issue. Could it be that with a button that's just small enough, a drag operation is triggered by the incident sliding of the finger when you tap the button with too much force?
It makes it pretty frustrating for users to get around some of my buttons. Obviously I can make them bigger and hopefully that address the issue, but it doesn't seem like the way it should work - not to mention if they pressed the button near the edges, they might still run into the issue if their finger slides off the edge of the button unwittingly.