In my opinion, saying that players can just turn it off if they don't like it is a crutch argument. If it works as you say and all the problems others have raised are worked out, then it gives an advantage, however small, with no drawbacks. Players trying to be as good as possible have no reasonable choice but to enable it, for them it may as well be mandatory.
I think there would need to be a very good reason to implement this besides ease of use. As Zlo has mentioned, aside from the added performance cost of having to check every idle/stationary engineer, something like this is sure to introduce plenty of bugs. Not to mention it would probably be incredibly difficult to develop. Yes it's not a "big feature", but it seriously messes with the way unit orders and reclaim works. Not to mention a feature like this is sure to create a ton of confusion on how it works with a name like "auto reclaim", even if it's a relatively simple feature.