editings (ex. in-/ out-points) are saved/synchronized to the timeline item
You do not need to open a timeline clip in the Source player to adjust it. Not only can you trim using the drag handles but also the ‘I’, ‘O’, ‘shift+I’ and ‘shift+O’ keyboard shortcuts work in Timeline.
Today, double-clicking a timeline clip selects the clip and seeks to its starting frame. Copying it to the Source player, which also acts like a clipboard, is as simple as Edit > Copy, Copy toolbar button, or Ctrl+C.
Opening something in the Source player needs to be very deliberate because you might have some trimmed and filtered clip staged in there that you do not want to lose (and I do not like frequent modal dialogs impeding my workflow).