Why I cannot extend value column for plist files? This is what I see and cannot extend the value column at all. Why is it so? Is it a bug or am I missing something?
It's a bug. The Value column is meant to be the remaining width in the window.
You can fix it by dragging the right side of the window small enough to hide some of the Value column, then dragging the window bigger again. When you drag the window bigger this time, the right edge of the Value column will follow the side of the window.