Yes, it is. Think about sending units around in an RTS Game. The player clicks, and nothing happens. He clicks again at the same location, and again nothing happens. The Unit would never move, unless the player is moving the mouse to another location. This is extremely frustrating for the player, isn’t it?
A slightly offset may produce large differences in where the trace hit’s the landscape. Think about a mountain in the foreground and some other stuff in the background. The player might click the edge of a mountain, but if the trace would be 1 pixel to the right, the trace may hit the background.