How about a relatively simple system, which combines many of the previous ideas? Normally, everything works without any movement indicators (as things are now).
You can press a button to show all the little flags of unit destinations (like Ryth's suggestion). Letting go of the button makes the flags disappear again - this way, it's very easy to check all unit destinations and avoid sending two settlers to the same spot.
Additionally, if you want to see the path of just one unit, either a right-click option or an action like CTRL+clicking (just an idea) on it will show its path (a line similar to the one you see when you move a unit by clicking and holding on a square to order a unit to move there). This will allow you to quickly answer the question "where is that worker going?".
Plus, the "normal" map remains uncluttered, and the movement system is unaffected...